Semiconductor device

ABSTRACT

Provided is a semiconductor device which can operate stably even in the case where a transistor thereof is a depletion transistor. The semiconductor device includes a first transistor for supplying a first potential to a first wiring, a second transistor for supplying a second potential to the first wiring, a third transistor for supplying a third potential at which the first transistor is turned on to a gate of the first transistor and stopping supplying the third potential, a fourth transistor for supplying the second potential to the gate of the first transistor, and a first circuit for generating a second signal obtained by offsetting a first signal. The second signal is input to a gate of the fourth transistor. The potential of a low level of the second signal is lower than the second potential.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/468,135, filed May 10, 2012, now allowed, which claims the benefit ofa foreign priority application filed in Japan as Serial No. 2011-108133on May 13, 2011, both of which are incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to semiconductor devicesand display devices.

2. Description of the Related Art

Display devices with higher value have been developed accompanyingspread of large-sized display devices such as liquid crystal television.In particular, a technology of using transistors whose conductivitytypes are the same as each other in a driver circuit has been activelydeveloped (see Patent Document 1).

FIG. 23 illustrates a driver circuit described in Patent Document 1. Thedriver circuit described in Patent Document 1 includes transistors M1,M2, M3, and M4 and a capacitor C1. In Patent Document 1, in the casewhere a signal at high level is output as a signal OUT, a gate of thetransistor M1 is made into a floating gate, and a bootstrap operation inwhich the potential of the gate of the transistor M1 is increased to behigher than a potential VDD by using capacitive coupling of thecapacitor C1. To make the gate of the transistor M1 into the floatinggate, a transistor (e.g., the transistor M4) connected to the gate ofthe transistor M1 is turned on by making the potential differencebetween gate and source of the transistor (hereinafter, the differenceis referred to as Vgs) 0 V.

Further, in the case where a signal at low level is output as the signalOUT, a signal at high level is input as a signal IN, and thus thetransistors M2 and M3 are turned on.

REFERENCE

-   Patent Document 1: Japanese Published Patent Application No.    2002-328643

SUMMARY OF THE INVENTION

When a depletion transistor (also called a normally-on transistor) isused as the transistor, the transistor is not turned off at Vgs of 0 V.Therefore, in the case where the signal at high level is output as thesignal OUT, the transistors M3 and M4 are not turned off, and thus thegate of the transistor M1 is not made into the floating gate. When thegate of the transistor M1 cannot be made into the floating state, thebootstrap operation cannot be performed normally, which may lead tomalfunction or narrowing of the operating frequency range.

Further, in the case where the signal at low level is output as thesignal OUT, since the driving voltage of the driver circuit of a displaydevice is high, Vgs of the transistor M2 and the transistor M3 are alsolarge, which promotes degradation of the transistors and may causemalfunction of the driver circuit.

In view of the above, one object of one embodiment of the presentinvention is to provide a semiconductor device which can operate stablyeven in the case where a transistor thereof is a depletion transistor.Further, one object of one embodiment of the present invention is tosuppress degradation of a transistor.

A semiconductor device of one embodiment of the present inventionincludes a first transistor for supplying a first potential to a firstwiring, a second transistor for supplying a second potential to thefirst wiring, a third transistor for supplying a third potential atwhich the first transistor is turned on to a gate of the firsttransistor and stopping supplying the third potential, a fourthtransistor for supplying the second potential to the gate of the firsttransistor, and a first circuit for generating a second signal obtainedby offsetting a first signal. The second signal is input to a gate ofthe fourth transistor. The potential of a low level of the second signalis lower than the second potential.

A semiconductor device of one embodiment of the present inventionincludes a first transistor for supplying a first potential to a firstwiring, a second transistor for supplying a second potential to thefirst wiring, a third transistor for supplying a third potential atwhich the first transistor is turned on to a gate of the firsttransistor and stopping supplying the third potential, a fourthtransistor for supplying the second potential to the gate of the firsttransistor, a capacitor whose one electrode is input with a firstsignal, and a fifth transistor for supplying a fourth potential to theother electrode of the capacitor. A gate of the fourth transistor isconnected to the other electrode of the capacitor. The fourth potentialis lower than the second potential.

The first signal may be input to a gate of the second transistor in theabove-described semiconductor device.

According to one embodiment of the present invention, even in the casewhere a transistor is a depletion transistor, the transistor can beturned off. Further, the drain current of a transistor in the off-statecan be decreased. Accordingly, malfunction of a circuit can beprevented. Further, according to one embodiment of the presentinvention, Vgs of a transistor can be decreased, whereby degradation ofthe transistor can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are diagrams for illustrating a semiconductor deviceaccording to one embodiment of the present invention;

FIGS. 2A and 2B are diagrams for illustrating semiconductor devicesaccording to embodiments of the present invention;

FIGS. 3A and 3B are diagrams for illustrating semiconductor devicesaccording to embodiments of the present invention;

FIGS. 4A and 4B are diagrams for illustrating semiconductor devicesaccording to embodiments of the present invention;

FIGS. 5A and 5B are diagrams for illustrating semiconductor devicesaccording to embodiments of the present invention;

FIGS. 6A and 6B are diagrams for illustrating semiconductor devicesaccording to embodiments of the present invention;

FIG. 7 is a chart for illustrating a semiconductor device according toone embodiment of the present invention;

FIGS. 8A and 8B are diagrams for illustrating semiconductor devicesaccording to embodiments of the present invention;

FIGS. 9A and 9B are diagrams for illustrating semiconductor devicesaccording to embodiments of the present invention;

FIGS. 10A to 10C are diagrams for illustrating a semiconductor deviceaccording to one embodiment of the present invention;

FIG. 11 is a diagram for illustrating a shift register according to oneembodiment of the present invention;

FIG. 12 is a chart for illustrating a shift register according to oneembodiment of the present invention;

FIGS. 13A to 13C are views for illustrating display devices according toembodiments of the present invention;

FIGS. 14A to 14E are diagrams illustrating structures of oxide materialsaccording to embodiments of the present invention;

FIGS. 15A to 15C are diagrams illustrating a structure of an oxidematerial according to one embodiment of the present invention;

FIGS. 16A to 16C are diagrams illustrating a structure of an oxidematerial according to one embodiment of the present invention;

FIGS. 17A to 17D are diagrams illustrating structures of transistorsaccording to embodiments of the present invention;

FIGS. 18A to 18C are graphs each showing characteristics of a transistorusing an oxide semiconductor layer;

FIG. 19 is a graph showing a relation between off-state current andmeasuring substrate temperature of a transistor;

FIGS. 20A to 20D are views illustrating electronic devices according toembodiments of the present invention;

FIGS. 21A to 21D are views illustrating electronic devices according toembodiments of the present invention;

FIGS. 22A and 22B are diagrams for illustrating semiconductor devicesaccording to embodiments of the present invention; and

FIG. 23 is a diagram illustrating a conventional driver circuit.

DETAILED DESCRIPTION OF THE INVENTION

Examples of embodiments of the present invention are described withreference to the drawings below. Note that it will be readilyappreciated by those skilled in the art that details of the embodimentscan be modified in various ways without departing from the spirit andscope of the present invention. The present invention is therefore notlimited to the following description of the embodiments.

Embodiment 1

In this embodiment, one example of a semiconductor device which isdriven by a signal generated by offsetting an input signal is described.

A configuration of a semiconductor device of this embodiment isdescribed with reference to FIG. 1A. FIG. 1A is a circuit diagram of thesemiconductor device of this embodiment. The semiconductor device inFIG. 1A includes a circuit 100 and a circuit 110. The circuit 100 isconnected to a wiring 11, a wiring 12, a wiring 14, and the circuit 110.The circuit 110 is connected to a wiring 15, a wiring 13, a wiring 16,and the circuit 100. The wirings and the like connected to any of thecircuits 100 and 110 can be changed as appropriate depending on theconfigurations of the circuits 100 and 110.

Note that in this specification, the case where X and Y are electricallyconnected to each other, the case where X and Y are functionallyconnected to each other, and the case where X and Y are directlyconnected to each other are encompassed in the explicit description of“X is connected to Y”.

Potential VL1 is supplied to the wiring 13. The potential VL1 is apredetermined potential. The wiring 13 transmits the potential VL1.

Potential VL2 is supplied to the wiring 14. The potential VL2 is apredetermined potential and is lower than the potential VL1. The wiring14 transmits the potential VL2.

Potential VH is supplied to the wiring 15. The potential VH is apredetermined potential and is higher than the potential VL1. The wiring15 transmits the potential VH.

The wirings 13, 14, and 15 are also called power supply lines. Thepotentials VL1, VL2, and VH are also called power supply potentials andeach supplied from a power supply circuit or the like.

Signal IN is input to the wiring 11. The signal IN is an input signal ofthe semiconductor device. The signal IN is a digital signal whosehigh-level potential is VH and low-level potential is VL1. That is,either the potential VH or the potential VL1 is supplied to the wiring11. The wiring 11 transmits the signal IN.

Signal SE is input to the wiring 12. The signal SE is a signal forcontrolling the timing at which an offset voltage is generated. Thesignal SE is a digital signal whose high-level potential is higher thanVL2 and low-level potential is lower than or equal to VL2. That is,either the potential higher than the potential VL2 or the potentiallower than or equal to the potential VL2 is supplied to the wiring 12.The wiring 12 transmits the signal SE.

Signal OUT is output from the wiring 16. The signal OUT is an outputsignal of the semiconductor device. The signal OUT is a digital signalwhose high-level potential is VH and low-level potential is VL1. Thewiring 16 transmits the signal OUT.

The wirings 11, 12, and 16 are also called signal lines. Further, thesignal 1N, the signal SE, and the signal OUT are also called an inputsignal, a control signal, and an output signal, respectively.

The circuit 100 generates a signal INO by offsetting the signal IN. Thatis, the circuit 100 generates the signal INO which is less than thepotential of the signal IN by the offset voltage. The circuit 100outputs the signal INO to the circuit 110.

The low-level potential of the signal INO is lower than the potentialVL1 of the wiring 13. On the other hand, the high-level potential of thesignal INO is preferably higher than VL1 and lower than VH.

The circuit 110 selects the high level or the low level of the signalOUT in response to the signal INO (output signal of the circuit 100).For example, in the case where the circuit 110 is an inverter circuit,the circuit 110 outputs the low-level potential of the signal OUT whenthe signal INO is at the high level, whereas outputs the high-levelpotential of the signal OUT when the signal INO is at the low level. Thecircuit 110 selects which of the potential of the wiring 15 and thepotential of the wiring 13 is output to the wiring 16, in response tothe signal INO. For example, the circuit 110 outputs the potential ofthe wiring 13 to the wiring 16 when the signal INO is at the high level,whereas outputs the potential of the wiring 15 to the wiring 16 when thesignal INO is at the low level. The circuit 110 also increases thehigh-level potential of the signal OUT to the potential VH of the wiring15 by a bootstrap operation.

Next, a specific example of the circuit 100 and the circuit 110 isdescribed with reference to FIG. 1A.

The circuit 100 includes a capacitor 101 and a transistor 102. Oneelectrode of the capacitor 101 is connected to the wiring 11. A firstterminal (one of a source and a drain) of the transistor 102 isconnected to the wiring 14, a second terminal thereof is connected tothe other electrode of the capacitor 101, and a gate thereof isconnected to the wiring 12.

The circuit 110 includes transistors 111, 112, 113, and 114. A firstterminal of the transistor 111 is connected to the wiring 15, and asecond terminal thereof is connected to the wiring 16. A first terminalof the transistor 112 is connected to the wiring 13, a second terminalthereof is connected to the wiring 16, and a gate thereof is connectedto a gate of the transistor 114. A first terminal of the transistor 113is connected to the wiring 15, a second terminal thereof is connected toa gate of the transistor 111, and a gate thereof is connected to thewiring 15. A first terminal of the transistor 114 is connected to thewiring 13, a second terminal thereof is connected to the gate of thetransistor 111, and the gate thereof is connected to the other electrodeof the capacitor 101. A portion at which the gate of the transistor 111is connected to another transistor (e.g. transistor 113, transistor 114)is denoted by a node N1.

The capacitor 101 holds a potential difference between the wiring 11 andthe second terminal of the transistor 102. Thus, in the case where thesecond terminal of the transistor 102 is in the floating state, thepotential of the second terminal of the transistor 102 varies inaccordance with the signal input to the wiring 11, i.e., the potentialof the signal INO varies in accordance with the signal IN.

The transistor 102 supplies the potential VL2 of the wiring 14 to theother electrode of the capacitor 101. The timing at which the transistor102 supplies the potential VL2 to the other electrode of the capacitor101 is controlled by the signal SE of the wiring 12.

The potential supplied to the other electrode of the capacitor 101 bythe transistor 102 is lower than the potential VL1. Specifically, thetransistor 102 supplies a potential lower than the potential of thefirst terminal of the transistor 114 to the other electrode of thecapacitor 101.

The transistor 111 supplies the potential VH of the wiring 15 to thewiring 16. The transistor 111 also holds a potential difference betweenthe gate and the second terminal of the transistor 111. Thus, in thecase where the node N1 is in the floating state, the potential of thenode N1 increases as the potential of the wiring 16 increases.

In the case where a signal is input to the wiring 15, the transistor 111supplies the signal of the wiring 15 to the wiring 16.

The transistor 112 supplies the potential VL1 of the wiring 13 to thewiring 16. The timing at which the transistor 112 supplies the potentialVL1 to the wiring 16 is controlled by the signal INO (potential of theother electrode of the capacitor 101) output from the circuit 100.

The transistor 113 supplies the potential VH of the wiring 15 to thegate of the transistor 111. After the potential VH is supplied to thegate of the transistor 111, the transistor 113 stops supplying thepotential VH to the gate of the transistor 111. The transistor 113 keepssupplying the potential VH to the gate of the transistor 111 after thetransistor 111 is turned on until the transistor 113 is turned off.

The potential supplied to the gate of the transistor 111 by thetransistor 113 is a potential at which the transistor 111 is turned on.

The transistor 114 supplies the potential VL1 of the wiring 13 to thegate of the transistor 111. The timing at which the transistor 114supplies the potential VL1 to the gate of the transistor 111 iscontrolled by the signal INO output from the circuit 100.

The conductivity types of the transistors included in the semiconductordevice of this embodiment (e.g., transistors 102, 111, 112, 113, and114) are the same as each other. Description is made in this embodimentin the case where the transistors included in the semiconductor deviceof this embodiment are n-channel transistors.

Next, an example of a driving method of the semiconductor device shownin FIG. 1A is described with reference to FIG. 1B. FIG. 1B is an exampleof a timing chart for describing the driving method of the semiconductordevice shown in FIG. 1A.

A period is divided into a period T0 and a period T1 for description ofthe driving method of the semiconductor device shown in FIG. 1A.

The period T0 is a period for holding an offset voltage in the capacitor101. First, the signal IN is set at a low level, so that the potentialof the one electrode of the capacitor 101 becomes VL1. Further, thesignal SE is set at a high level to turn on the transistor 102.Consequently, the potential VL2 of the wiring 14 is supplied to theother electrode of the capacitor 101, so that the potential of the otherelectrode of the capacitor 101 becomes VL2. In this manner, a differencebetween the low-level potential VL1 of the signal IN and the potentialVL2 of the wiring 14 supplied through the transistor 102, i.e., thedifference (VL1−VL2), is held in the capacitor 101. The difference(VL1−VL2) corresponds to the offset voltage.

In the period T0, a potential lower than VL1 is supplied to the otherelectrode of the capacitor 101 through the transistor 102.

The period T1 is a period for generating the signal INO by offsettingthe signal IN and driving the circuit 110 by the signal INO. First, thesignal SE is changed to a low level to turn off the transistor 102,whereby the other electrode of the capacitor 101 is made into a floatingstate. Since the capacitor 101 holds the potential difference (VL1−VL2)in the period T0, a signal obtained by subtracting the potentialdifference (VL1−VL2) from the potential of the signal IN is generated asthe signal INO. Therefore, when the signal IN is at the low level, thesignal INO becomes a low level whose potential is lower than VL1; whenthe signal IN is at the high level, the signal INO becomes a high levelwhose potential is lower than VH.

The driving method of the semiconductor device shown in FIG. 1A in theperiod T1 is described with the case where the signal IN is at the highlevel and the case where the signal IN is at the low level.

In the period T1, when the potential of the signal IN is changed to thehigh level, the signal INO becomes the high level, so that thetransistors 112 and 114 are turned on. Consequently, the potential VL1of the wiring 13 is supplied to the wiring 16 through the transistor112. The potential VL1 of the wiring 13 is also supplied to the node N1through the transistor 114. The potential VH of the wiring 15 is alsosupplied to the node N1 through the transistor 113. However, thepotential of the node N1 becomes as low as a potential at which thetransistor 111 is turned off, where the W (channel width)/L (channellength) ratio of the transistor 114 is sufficiently larger than that ofthe transistor 113; thus, the transistor 111 is turned off. Accordingly,the signal OUT becomes a low-level potential which is VL1.

On the other hand, in the period T1, when the level of the signal IN ischanged to the low level, the signal INO becomes the low level, so thatthe transistors 112 and 114 are turned off. Since the potential VH ofthe wiring 15 is supplied to the node N1 through the transistor 113, thepotential of the node N1 increases. Consequently, the transistor 111 isturned on, so that the potential VH of the wiring 15 is supplied to thewiring 16 through the transistor 111, thereby increasing the potentialof the wiring 16. Then, the potential of the node N1 reaches a potentialobtained by subtracting the threshold voltage of the transistor 113 fromthe potential VH, so that the transistor 113 is turned off to make thenode N1 in a floating state. Even after the node N1 is made in thefloating state, the potential of the wiring 16 increases. In addition, apotential difference between the node N1 and the wiring 16 at the timewhen the transistor 113 is turned off is held between the gate and thesecond terminal of the transistor 111. Therefore, the potential of thenode N1 further increases to be higher than the potential VH along withthe increase in the potential of the wiring 16. The above is a so-calledbootstrap operation. Accordingly, the signal OUT becomes a high-levelpotential which is VH.

In the case where a signal is input to the wiring 15, the signal isoutput to the wiring 16. For example, in the case where a clock signalis input to the wiring 15, the clock signal is output to the wiring 16from the wiring 15 in the period during which the signal IN is at thelow level.

As described above, when the signal OUT is at the high level, thepotential of the gate of the transistor 114 is lower than VL1, and thusVgs of the transistor 114 is a negative value; therefore, even if thetransistor 114 is a depletion transistor, the transistor 114 can beturned off, or even if the transistor 114 is a transistor whose draincurrent at Vgs of 0 V is large, the drain current of the transistor 114can be suppressed. Accordingly, the gate of the transistor 111 can bemade into the floating state, whereby malfunction of the circuit 110 canbe prevented.

Further, like the transistor 114, Vgs of the transistor 112 is also anegative value. Therefore, even if the transistor 112 is a depletiontransistor, the transistor 112 can be turned off, or even if thetransistor 112 is a transistor whose drain current at Vgs of 0 V islarge, the drain current of the transistor 112 can be suppressed.Accordingly, current flow from the wiring 16 to the wiring 13 can beprevented or suppressed, by which power consumption can be reduced.

Further, when the signal OUT is at the low level, the potentials of thegates of the transistors 112 and 114 are lower than VH, and thus Vgs ofthe transistors 112 and are small. Accordingly, degradation of thetransistors 112 and 114 can be suppressed.

Heretofore, the driving method of the semiconductor device shown in FIG.1A is described.

Next, semiconductor devices different from FIG. 1A are described withreference to FIGS. 2A, 2B, 3A, 3B, 4A, 4B, 5A, and 5B. Description ismade on portions different from FIG. 1A, below.

As shown in FIG. 2A, the wiring 14 in the semiconductor device shown inFIG. 1A can be omitted, and the first terminal of the transistor 102 maybe connected to the wiring 13. Then, the potential VL2 may be suppliedto the wiring 13 in the period T0 and the potential VL1 may be suppliedto the wiring 13 in the period T1. Even in that case, the potential VL2can be supplied to the other electrode of the capacitor 101 in theperiod T0, and thus an operation similar to that of the semiconductordevice shown in FIG. 1A can be performed. Accordingly, an effect similarto that in the semiconductor device shown in FIG. 1A can be attained.Further, since the wiring 14 can be omitted, the number of wirings canbe reduced as compared to that of the semiconductor device shown in FIG.1A.

Further, in the semiconductor device shown in FIG. 2A, the potential ofthe wiring 13 may not be changed but remain the potential VL1 and thepotential of the wiring 11 may be set to be higher than the potentialVL1 and lower than the potential VH in the period T0. Even in that case,the potential of the other electrode of the capacitor 101 can become apotential lower than the potential VL1 when the signal IN is at the lowlevel in the period T1, and thus an operation similar to that of thesemiconductor device shown in FIG. 1A can be performed. Accordingly, aneffect similar to that in the semiconductor device shown in FIG. 1A canbe attained. Further, since the power supply potential can be fixed, aconfiguration of a power supply circuit to supply the potential to thewiring 13, or the like can be simplified.

As shown in FIG. 2B, the wiring 14 in the semiconductor device shown inFIG. 1A can be omitted, and the first terminal of the transistor 102 maybe connected to the wiring 15. Then, the potential VL2 may be suppliedto the wiring 15 in the period T0 and the potential VH may be suppliedto the wiring 15 in the period T1. Even in that case, the potential VL2can be supplied to the other electrode of the capacitor 101 in theperiod T0, and thus an operation similar to that of the semiconductordevice shown in FIG. 1A can be performed. Accordingly, an effect similarto that in the semiconductor device shown in FIG. 1A can be attained.Further, since the wiring 14 can be omitted, the number of wirings canbe smaller than that in the semiconductor device shown in FIG. 1A.

As shown in FIG. 3A, the wiring 14 in the semiconductor device shown inFIG. 1A can be omitted, and the first terminal of the transistor 102 maybe connected to the wiring 12 and the second terminal and the gate ofthe transistor 102 may be connected to the other electrode of thecapacitor 101. Then, the signal SE may be set at the low level in theperiod T0 and at the high level in the period T1. Even in that case, thepotential of the other electrode of the capacitor 101 can become apotential lower than the potential VL1 in the period T0, and thus anoperation similar to that of the semiconductor device shown in FIG. 1Acan be performed. Accordingly, an effect similar to that in thesemiconductor device shown in FIG. 1A can be attained. Further, sincethe wiring 14 can be omitted, the number of wirings can be smaller thanthat in the semiconductor device shown in FIG. 1A.

As shown in FIG. 3B, the wirings 12 and 14 in the semiconductor deviceshown in FIG. 1A can be omitted, and the first terminal of thetransistor 102 may be connected to the wiring 13 and the second terminaland the gate of the transistor 102 may be connected to the otherelectrode of the capacitor 101. Then, the potential VL2 may be suppliedto the wiring 13 in the period T0 and the potential VL1 may be suppliedto the wiring 13 in period T1. Even in that case, the potential of theother electrode of the capacitor 101 can become a potential lower thanthe potential VL1 in the period T0, and thus an operation similar tothat of the semiconductor device shown in FIG. 1A can be performed.Accordingly, an effect similar to that in the semiconductor device shownin FIG. 1A can be attained. Further, since the wirings 12 and 14 can beomitted, the number of wirings can be smaller than that in thesemiconductor device shown in FIG. 1A.

As shown in FIG. 4A, the wirings 12 and 14 in the semiconductor deviceshown in FIG. 1A can be omitted, and the first terminal of thetransistor 102 may be connected to the wiring 15 and the second terminaland the gate of the transistor 102 may be connected to the otherelectrode of the capacitor 101. Then, the potential VL2 may be suppliedto the wiring 15 in the period T0 and the potential VH may be suppliedto the wiring 15 in period T1. Even in that case, the potential of theother electrode of the capacitor 101 can become a potential lower thanthe potential VL1 in the period T0, and thus an operation similar tothat of the semiconductor device shown in FIG. 1A can be performed.Accordingly, an effect similar to that in the semiconductor device shownin FIG. 1A can be attained. Further, since the wirings 12 and 14 can beomitted, the number of wirings can be smaller than that in thesemiconductor device shown in FIG. 1A.

As shown in FIG. 4B, the gate of the transistor 112 may be connected tothe wiring 11 in the semiconductor device shown in FIG. 1A. In thesemiconductor device shown in FIG. 4B, the timing at which the potentialVL1 of the wiring 13 is supplied to the wiring 16 through the transistor112 is controlled by the signal IN. Since the signal IN rises or fallsfaster than the signal INO, the transistor 112 can be turned on or offsooner than the case where the gate of the transistor 112 is connectedto the other electrode of the capacitor 101. Accordingly, the timing atwhich the potential VL1 of the wiring 13 is supplied to the wiring 16becomes sooner, so that the fall time of the signal OUT can be reduced.Further, as the timing at which the transistor 112 is turned off getssooner, the time during which a flow-through current between the wirings15 and 13 flows can be shortened, whereby power consumption can bereduced.

Like the semiconductor device shown in FIG. 4B, the gate of thetransistor 112 may be connected to the wiring 11 also in anysemiconductor device shown in FIGS. 2A, 2B, 3A, 3B, and 4A. Also in thatcase, an effect similar to that in the semiconductor device shown inFIG. 4B can be attained.

As shown in FIG. 5A, a transistor 115 whose first terminal is connectedto the wiring 13, second terminal is connected to the gate of thetransistor 111, and gate is connected to the wiring 12 may be providedin the semiconductor device shown in FIG. 1A. The potential VL1 of thewiring 13 is supplied to the gate of the transistor 111 through thetransistor 115. The timing at which the potential VL1 is supplied to thegate of the transistor 111 through the transistor 115 is controlled bythe signal SE of the wiring 12. In the semiconductor device shown inFIG. 5A, the potential VL1 of the wiring 13 can be supplied to the gateof the transistor 111 in the period T0, whereby the semiconductor devicecan be initialized. Accordingly, malfunction of the semiconductor devicecan be prevented.

Further, in the semiconductor device shown in FIG. 5A, the firstterminal of the transistor 115 may be connected to the wiring 14. Evenin that case, an operation similar to that in the case where the firstterminal of the transistor 115 is connected to the wiring 13 can beperformed.

In the case where the timing at which the offset voltage is generateddoes not coincide with the timing at which initialization is performed,the gate of the transistor 115 may be connected to the wiring to which asignal for initialization is input.

The transistor 115 whose first terminal is connected to the wiring 13 orthe wiring 14, second terminal is connected to the gate of thetransistor 111, and gate is connected to the wiring 12 may be providedalso in any semiconductor device shown in FIGS. 2A, 2B, 3A, 3B, 4A, and4B. Also in that case, an effect similar to that in the semiconductordevice shown in FIG. 5A can be attained.

As shown in FIG. 5B, the second terminal and the gate of the transistor113 may be connected to a wiring 17 in the semiconductor device shown inFIG. 1A. To the wiring 17, the potential VH, a potential higher than thepotential VL1 and lower than the potential VH, or a signal may besupplied. An example of the signal which is input to the wiring 17 is aninverted signal of the signal IN. Therefore, the wiring 11 may beconnected to the wiring 17 with an inverter provided therebetween. Inthat case, the transistor 113 is turned off when the transistor 114 isturned on, whereby current can be prevented from flowing between thewiring 15 and the wiring 13. Thus, power consumption can be reduced.Further, there is no need to make the W/L ratio of the transistor 114sufficiently larger than that of the transistor 113, leading to areduction in size of the transistor.

The second terminal and the gate of the transistor 113 may be connectedto the wiring 17 also in any semiconductor device shown in FIGS. 2A, 2B,3A, 3B, 4A, 4B, and FIG. 5A. Also in that case, an effect similar tothat in the semiconductor device shown in FIG. 5B can be attained.

As shown in FIG. 22A, the wiring 14 in the semiconductor device shown inFIG. 1A can be omitted, and the first terminal of the transistor 102 maybe connected to the wiring 13, and a capacitor 103 one electrode ofwhich is connected to the wiring 12 and the other electrode of which isconnected to the other electrode of the capacitor 101 may be provided.The capacitor 103 holds a potential difference between the wiring 12 andthe other electrode of the capacitor 101. Further, in the semiconductordevice shown in FIG. 22A, the potential VL1 of the wiring 13 is suppliedto the other electrode of the capacitor 101 through the transistor 102.In the semiconductor device shown in FIG. 22A, in the period T0, thesignal IN at the low level is input to the one electrode of thecapacitor 101, and the potential VL1 of the wiring 13 is supplied to theother electrode of the capacitor 101 through the transistor 102. Then,the signal SE is changed from the high level to the low level, so thatthe transistor 102 is turned off, and thus the potential of the otherelectrode of the capacitor 101 becomes a potential lower than thepotential VL1 owing to the capacitive coupling with the capacitor 103.Accordingly, the potential of the other electrode of the capacitor 101becomes the potential lower than the potential VL1 in the period T0, andthus an operation similar to that of the semiconductor device shown inFIG. 1A can be performed. Accordingly, an effect similar to that in thesemiconductor device shown in FIG. 1A can be attained. Further, sincethe wiring 14 can be omitted, the number of wirings can be smaller thanthat in the semiconductor device shown in FIG. 1A. In addition, sincethe potential VL2 is not used, the number of power supply potentials canbe reduced.

As shown in FIG. 22B, the first terminal of the transistor 102 may beconnected to the wiring 11 in the semiconductor device shown in FIG.22A. Also in that case, in the period T0, the signal IN at the low levelcan be supplied to the other electrode of the capacitor 101 through thetransistor 102, and thus an operation similar to that of thesemiconductor device shown in FIG. 22A can be performed. Accordingly, aneffect similar to that in the semiconductor device shown in FIG. 22A canbe attained.

The capacitor 103 may be omitted in any semiconductor device shown inFIGS. 22A and 22B. In that case, parasitic capacitance between the gateand the second terminal of the transistor 102 may be used instead of thecapacitor 103.

In any semiconductor device shown in FIGS. 22A and 22B, the oneelectrode of the capacitor 103 may be connected to a wiring other thanthe wiring 12. It is preferable that a signal input to the wiring ischanged from a high level to a low level after the signal SE is changedfrom the high level to the low level in the period T0. This is becausethe potential of the other electrode of the capacitor 101 can bedecreased after the transistor 102 is turned off. Further, it ispreferable that the timing at which the signal input to the wiring ischanged from the low level to the high level is in the period duringwhich the signal SE is at the high level.

Also in any semiconductor device shown in FIGS. 2A, 2B, 3A, 3B, 4A, 4B,5A, and 5B, the wiring 14 can be omitted, and the first terminal of thetransistor 102 may be connected to the wiring 11 or the wiring 13, andthe capacitor 103 one electrode of which is connected to the wiring 12and the other electrode of which is connected to the other electrode ofthe capacitor 101 may be provided.

Further, a capacitor may be connected between the gate and the secondterminal of the transistor 111 in any semiconductor device shown inFIGS. 2A, 2B, 3A, 3B, 4A, 4B, 5A, 5B, 22A, and 22B, though not shown.Thus, the capacitance value between the wiring 16 and the node N1 can beincreased. Accordingly, in the period during which the signal IN is atthe low level, the potential of the node N1 can become a higherpotential than the potential of the node N1 in the case where nocapacitor is provided between the gate and the second terminal of thetransistor 111. That is, Vgs of the transistor 111 can be increased. Thedrain current of the transistor 111 can be increased accordingly, whichshorten the rise time of the signal OUT.

Further, a MOS capacitor may be used as the capacitor 101 in anysemiconductor device shown in FIGS. 2A, 2B, 3A, 3B, 4A, 4B, 5A, 5B, 22A,and 22B, though not shown. In that case, it is preferable that a gate ofthe transistor used as the MOS capacitor is connected to the wiring 11,and a source or a drain of the transistor is connected to the secondterminal of the transistor 102. Thus, the capacitance value per unitarea can be increased because the potential of the wiring 11 is higherthan that of the second terminal of the transistor 102.

Heretofore, the semiconductor devices whose configurations are differentfrom FIG. 1A are described.

As the W/L ratio of the transistor 111 increases, the rise time of thesignal OUT can be shortened. Therefore, it is preferable that the W/Lratio of the transistor 111 is the largest among the transistors in thesemiconductor device. That is, it is preferable that the W/L ratio ofthe transistor 111 is larger than any of those of the transistors 102,112, 113, and 114.

The transistor 112 supplies a potential to a load connected to thewiring 16, whereas the transistor 114 supplies a potential to the gateof the transistor 111. In addition, as the W/L ratio of the transistor112 increases, the fall time of the signal OUT can be shortened.Therefore, it is preferable that the W/L ratio of the transistor 112 islarger than that of the transistor 114.

On the other hand, there is no need to make the W/L ratio of thetransistor 102, which is used for supplying charge to the otherelectrode of the capacitor 101 in the period T0, large. Therefore, it ispreferable that the W/L ratio of the transistor 102 is smaller than thatof the transistor 112 or the transistor 114.

Further, as the capacitance value of the capacitor 101 gets larger thanthe total of the gate capacitance of the transistors 112 and 114, theamplitude voltage of the signal INO can get closer to that of the signalIN. Therefore, it is preferable that the capacitance value of thecapacitor 101 is larger than the total of the gate capacitance of thetransistors 112 and 114. Further or alternatively, when the oneelectrode of the capacitor 101 is formed of the same material as a gateelectrode of a transistor and the other electrode of the capacitor 101is formed of the same material as a source or drain electrode of thetransistor, it is preferable that the area where the two electrodes ofthe capacitor 101 are overlapped with each other is larger than the sumof the area where the gate and source of the transistor 112 areoverlapped with each other, the area where the gate and drain of thetransistor 112 are overlapped with each other, the area where the gateand source of the transistor 114 are overlapped with each other, and thearea where the gate and drain of the transistor 114 are overlapped witheach other.

In the period T0, the potential VL1 may not be supplied to the wiring13, so that the wiring 13 can be made in a floating state, or thepotential VH may not be supplied to the wiring 15, so that the wiring 15can be made in a floating state; thus, malfunction in the period T0 canbe prevented.

In the period T1, the potential VL2 may not be supplied to the wiring14, so that the wiring 14 can be made in a floating state.

A low-level signal may be input to the wiring 15 in the period duringwhich the signal IN is at the high level. In that case, the transistor113 is turned off when the transistor 114 is turned on, whereby currentcan be prevented from flowing between the wiring 15 and the wiring 13.Thus, power consumption can be reduced. Further, there is no need tomake the W/L ratio of the transistor 114 sufficiently larger than thatof the transistor 113, leading to a reduction in size of the transistor.

This embodiment can be implemented in appropriate combination with anyother embodiment and the like.

Embodiment 2

In this embodiment, the case where the semiconductor device which is oneembodiment of the present invention is used for a flip-flop circuitincluded in a shift register is described. Description is made onportions different from Embodiment 1.

The semiconductor device of this embodiment is described with referenceto FIG. 6A. FIG. 6A is a circuit diagram of the semiconductor device ofthis embodiment. The semiconductor device in FIG. 6A is different fromthat shown in FIG. 1A in that the first terminal of the transistor 111is connected to a wiring 23, the gate of the transistor 113 is connectedto a wiring 21, and the one electrode of the capacitor 101 is connectedto a wiring 22.

Signal IN1 is input to the wiring 21. The signal IN1 is an input signalof the semiconductor device and serves as a start pulse. For example,the signal IN1 is a digital signal whose high-level potential is VH andlow-level potential is VL1. The wiring 21 transmits the signal IN1.

Signal IN2 is input to the wiring 22. The signal IN2 is an input signalof the semiconductor device and serves as a reset signal. For example,the signal IN2 is a digital signal whose high-level potential is VH andlow-level potential is VL1. The wiring 22 transmits the signal IN2.

Signal CK is input to the wiring 23. The signal CK is an input signal ofthe semiconductor device. For example, the signal CK is a digital signalwhose high-level potential is VH and low-level potential is VL1. Thesignal CK is a clock signal whose potential is switched between the highlevel and the low level repeatedly. The wiring 23 transmits the signalCK.

The wirings 21, 22, and 23 are also called signal lines. In particular,the wiring 23 is also called a clock signal line.

Next, an example of a driving method of the semiconductor device shownin FIG. 6A is described with reference to FIG. 7. FIG. 7 is an exampleof a timing chart for describing the driving method of the semiconductordevice shown in FIG. 6A.

In a period T0, the signal IN2 is set at a low level, so that thepotential of the one electrode of the capacitor 101 becomes VL1.Further, the signal SE is set at the high level to turn on thetransistor 102. Consequently, the potential VL2 of the wiring 14 issupplied to the other electrode of the capacitor 101, so that thepotential of the other electrode of the capacitor 101 becomes VL2. Inthis manner, a difference between the low-level potential VL1 of thesignal IN2 and the potential VL2 of the wiring 14 supplied through thetransistor 102, i.e., the difference (VL1−VL2), is held in the capacitor101. The difference (VL1−VL2) corresponds to an offset voltage.

In a period T1, the signal SE is changed to the low level to turn offthe transistor 102, whereby the other electrode of the capacitor 101 ismade into the floating state. Since the capacitor 101 holds thepotential difference (VL1−VL2) in the period T0, a signal obtained bysubtracting the potential difference (VL1−VL2) from the potential ofsignal IN2 is generated as a signal IN2O. Therefore, when the signal IN2is at the low level, the signal IN2O becomes a low level whose potentialis lower than VL1; when the signal IN2 is at a high level, the signalIN2O becomes a high level whose potential is lower than VH.

The driving method of the semiconductor device shown in FIG. 6A in theperiod T1 is described separately for each of a period Ta, a period Tb,a period Tc, and a period Td.

In the period Ta, since the signal IN2 is at the low level, the signalIN2O becomes the low level, so that the transistors 112 and 114 areturned off. Further, the signal IN1 is changed to the high level, sothat the transistor 113 is turned on. Consequently, the potential VH ofthe wiring 15 is supplied to the node N1, and accordingly the potentialof the node N1 increases. Consequently, the transistor 111 is turned on,so that the signal CK of the wiring 23 is supplied to the wiring 16.Since the signal CK is at the low level in the period Ta, the signal OUTbecomes the low level whose potential is VL1. Then, the potential of thenode N1 reaches a potential obtained by subtracting the thresholdvoltage of the transistor 113 from the potential VH, so that thetransistor 113 is turned off to make the node N1 in the floating state.A potential difference between the node N1 and the wiring 16 at the timewhen the transistor 113 is turned off is held between the gate and thesecond terminal of the transistor 111.

In the period Tb, since the signal IN2 is kept at the low level, thesignal IN2O is also kept at the low level, so that the transistors 112and 114 are kept off. Further, the potential of the signal IN1 ischanged to the low level, so that the transistor 113 is kept off.Therefore, the node N1 is kept in the floating state. Since the node N1is kept at the potential in the period Ta, the transistor 111 is kepton, and thus the signal CK of the wiring 23 is kept supplied to thewiring 16. In the period Tb, the potential of the signal CK is changedto the high level, so that the potential of the wiring 16 increases. Inthe meantime, the potential difference between the node N1 and thewiring 16 in the period Ta is kept to be held between the gate and thesecond terminal of the transistor 111. Accordingly, the potential of thenode N1 further increases to be higher than the potential VH along withthe increase in the potential of the wiring 16. Accordingly, the signalOUT becomes the high-level potential which is VH.

In the period Tc, the level of the signal IN2 is changed to the highlevel, and thus the level of the signal IN2O is also changed to the highlevel, so that the transistors 112 and 114 are turned on. Consequently,the potential VL1 of the wiring 13 is supplied to the wiring 16 throughthe transistor 112 and to the node N1 through the transistor 114. On theother hand, the signal IN1 is kept at the low level, and thus thetransistor 113 is kept off. Accordingly, the potential of the node N1 ischanged to the potential VL1, so that the transistor 111 is turned off.Accordingly, the signal OUT becomes the low-level potential which isVL1.

In the period Td, the level of the signal IN2 is changed to the lowlevel, and thus the level of the signal IN2O is also changed to the lowlevel, so that the transistors 112 and 114 are turned off. Further, thesignal IN1 is kept at the low level, and thus the transistor 113 is keptoff. Accordingly, the node N1 is kept at the potential VL1 in the periodTc, so that the transistor 111 is kept off. Further, the wiring 16 iskept at the potential VL1 in the period Tc, so that the signal OUT iskept at the low-level potential.

As described above, when the signal IN2 is at the low level, thepotential of the gate of the transistor 114 is lower than VL1, and thusVgs of the transistor 114 is a negative value; therefore, even if thetransistor 114 is a depletion transistor, the transistor 114 can beturned off, or even if the transistor 114 is a transistor whose draincurrent at Vgs of 0 V is large, the drain current of the transistor 114can be suppressed. Accordingly, the gate of the transistor 111 can bemade into the floating state, whereby malfunction of the circuit 110 canbe prevented.

Further, when the signal IN2 is at the high level, the potentials of thegates of the transistors 112 and 114 are lower than VH, and thus Vgs ofthe transistors 112 and 114 are small. Accordingly, degradation of thetransistors 112 and 114 can be suppressed.

Heretofore, the driving method of the semiconductor device shown in FIG.6A is described.

Next, semiconductor devices different from FIG. 6A are described withreference to FIGS. 6B, 8A, 8B, 9A, 9B, and 10A. Description is made onportions different from FIG. 6A, below.

As shown in FIG. 6B, the first terminal of the transistor 113 may beconnected to the wiring 21 in the semiconductor device shown in FIG. 6A.In the semiconductor device shown in FIG. 6B, the signal IN1 of thewiring 21 is supplied to the node N1 through the transistor 113 in theperiod Ta. In the period Ta, since the signal IN1 is at the high level,the potential of the node N1 increases. Then, the potential of the nodeN1 reaches a potential obtained by subtracting the threshold voltage ofthe transistor 113 from the potential VH, so that the transistor 113 isturned off. The transistor 113 is kept off in the periods Tb, Tc, andTd. Accordingly, an operation similar to that of the semiconductordevice shown in FIG. 6A can be performed. Accordingly, an effect similarto that in the semiconductor device shown in FIG. 6A can be attained.Further, since the wiring 15 can be omitted, the number of wirings canbe smaller than that in the semiconductor device shown in FIG. 6A.

As shown in FIG. 8A, the circuit 100 in the semiconductor device shownin FIG. 6B may be connected to the wiring 21 instead of the wiring 22.In the semiconductor device shown in FIG. 8A, the signal IN1 of thewiring 21 connected to the circuit 100 is offset to generate a signalIN1O and the signal IN1O is supplied to the gate of the transistor 113.The one electrode of the capacitor 101 is connected to the wiring 21,and the other electrode thereof is connected to the gate of thetransistor 113. The first terminal of the transistor 102 is connected tothe wiring 14, the second terminal of the transistor 102 is connected tothe other electrode of the capacitor 101, and the gate of the transistor102 is connected to the wiring 12. The capacitor 101 holds a potentialdifference between the wiring 21 and the gate of the transistor 113, andthe transistor 102 supplies the potential VL1 of the wiring 14 to thegate of the transistor 113. In the semiconductor device shown in FIG.8A, Vgs of the transistor 113 can be suppressed to be negative;therefore, the W/L ratio of the transistor 113 can be made large withoutconsidering the amount of charge supplied to the node N1. Accordingly,the time the potential of the node N1 takes to reach the above-describedpotential can be shortened, increasing the driving frequency.

As shown in FIG. 8B, the circuit 100 may be provided not only for thewiring 22 but also for the wiring 21 in the semiconductor device shownin FIG. 6B. In FIG. 8B, the circuit 100 provided for the wiring 22, andthe capacitor 101 and the transistor 102 included in the circuit 100 aredenoted by a circuit 100A, a capacitor 101A, and a transistor 102A,respectively; the circuit 100 provided for the wiring 21, and thecapacitor 101 and the transistor 102 included in the circuit 100 aredenoted by a circuit 100B, a capacitor 101B, and a transistor 102B,respectively. The circuit 100A is similar to the circuit 100 shown inFIG. 6A, and the circuit 100B is similar to the circuit 100 shown inFIG. 8A; therefore, description thereof is skipped. An effect similar tothat in the semiconductor device shown in FIG. 6B and an effect similarto that in the semiconductor device shown in FIG. 8A can be attained bythe semiconductor device shown in FIG. 8B.

As shown in FIG. 9A, the gate of the transistor 112 may be connected toa wiring 24 in the semiconductor device shown in FIG. 6A. Signal IN3 isinput to the wiring 24. The wiring 24 transmits the signal IN3. Thesignal IN3 is a digital signal whose high-level potential is VH andlow-level potential is VL1. As examples of the signal IN3, there are aclock signal which is an inverted signal of the signal CK, a clocksignal whose phase is shifted from that of the signal CK, and the like.In the semiconductor device shown in FIG. 9A, in the period Td, thetransistor 112 is switched between on and off repeatedly, so that thepotential VL1 of the wiring 13 can be supplied to the wiring 16periodically, whereby the potential of the wiring 16 can be kept at VL1more surely.

The gate of the transistor 112 may be connected to the wiring 24 also inany semiconductor device shown in FIGS. 6B, 8A, and 8B. Also in thatcase, an effect similar to that in the semiconductor device shown inFIG. 9A can be attained.

A transistor whose first terminal is connected to the wiring 13, secondterminal is connected to the wiring 16, and gate is connected to thewiring 24 may be provided in any semiconductor device shown in FIGS. 6A,6B, 8A, and 8B. Also in that case, an effect similar to that in thesemiconductor device shown in FIG. 9A can be attained.

As shown in FIG. 9B, a transistor 116 whose first terminal is connectedto the wiring 23, second terminal is connected to a wiring 25, and gateis connected to the gate of the transistor 111 may be provided in thesemiconductor device shown in FIG. 6A. The transistor 116 supplies thesignal CK of the wiring 23 to the wiring 25. The timing at which thesignal CK of the wiring 23 is supplied to the wiring 25 is controlled bythe potential of the node N1. The transistor 116 also holds a potentialdifference between the wiring 25 and the node N1. The signal OUT isoutput from the wiring 25. The wiring 25 transmits the signal OUT. InFIG. 9B, the signal OUT output from the wiring 16 is denoted by a signalOUTA whereas the signal OUT output from the wiring 25 is denoted by asignal OUTB. The signal OUTA is switched between a high level and a lowlevel at the same timing as the signal OUTB. In the semiconductor deviceshown in FIG. 9B, one of the signal OUTA and the signal OUTB can be usedas a forward signal of the shift register and the other can be used as asignal for driving a load or the like. Accordingly, with thesemiconductor device shown in FIG. 9B used in the flip-flop circuit,normal operation can be performed even when a large load is driven.

The transistor 116 whose first terminal is connected to the wiring 23,second terminal is connected to the wiring 25, and gate is connected tothe gate of the transistor 111 may be provided also in any semiconductordevice shown in FIGS. 6B, 8A, 8B, and 9A. Also in that case, an effectsimilar to that in the semiconductor device shown in FIG. 9B can beattained.

As shown in FIG. 10A, a circuit 120 for generating the signal IN2 may beprovided in the semiconductor device shown in FIG. 6A. The circuit 120is connected to the node N1, the wiring 12, and the one electrode of thecapacitor 101. The circuit 120 generates the signal IN2 in accordancewith the potential of the node N1 and the signal SE of the wiring 12 andoutputs to the one electrode of the capacitor 101. For example, thecircuit 120 generates the signal IN2 at the low level when the signal SEis at the high level regardless of the potential of the node N1; andwhen the signal SE is at the low level, the circuit 120 generates thesignal IN2 at the low level when the potential of the node N1 is high(e.g., the period Ta, the period Tb) and generates the signal IN2 at thehigh level when the potential of the node N1 is low (e.g., the periodTc, the period Td). That is, the circuit 120 serves as a NOR circuit.

The circuit 120 may be connected to the wiring 16 instead of the nodeN1.

The circuit 120 for generating the signal IN2 may be provided also inany semiconductor device shown in FIGS. 6B, 8A, 8B, 9A, and 9B.

Although not shown in the drawing, the second terminal of the transistor102 may be connected to the wiring 13 also in any semiconductor deviceshown in FIGS. 6A, 6B, 8A, 8B, 9A, 9B, and 10A, like the semiconductordevice shown in FIG. 2A. Also in that case, an effect similar to that inthe semiconductor device shown in FIG. 2A can be attained.

Although not shown in the drawing, the second terminal of the transistor102 may be connected to the wiring 15 also in any semiconductor deviceshown in FIGS. 6A, 6B, 8A, 8B, 9A, 9B, and 10A, like the semiconductordevice shown in FIG. 2B. Also in that case, an effect similar to that inthe semiconductor device shown in FIG. 2B can be attained.

Although not shown in the drawing, the first terminal of the transistor102 may be connected to the wiring 12 and the gate of the transistor 102may be connected to the second terminal of the transistor 102 also inany semiconductor device shown in FIGS. 6A, 6B, 8A, 8B, 9A, 9B, and 10A,like the semiconductor device shown in FIG. 3A. Also in that case, aneffect similar to that in the semiconductor device shown in FIG. 3A canbe attained.

Although not shown in the drawing, the first terminal of the transistor102 may be connected to the wiring 13 and the gate of the transistor 102may be connected to the second terminal of the transistor 102 also inany semiconductor device shown in FIGS. 6A, 6B, 8A, 8B, 9A, 9B, and 10A,like the semiconductor device shown in FIG. 3B. Also in that case, aneffect similar to that in the semiconductor device shown in FIG. 3B canbe attained.

Although not shown in the drawing, the first terminal of the transistor102 may be connected to the wiring 15 and the gate of the transistor 102may be connected to the second terminal of the transistor 102 also inany semiconductor device shown in FIGS. 6A, 6B, 8A, 8B, 9A, 9B, and 10A,like the semiconductor device shown in FIG. 4A. Also in that case, aneffect similar to that in the semiconductor device shown in FIG. 4A canbe attained.

Although not shown in the drawing, the gate of the transistor 112 may beconnected to the one electrode of the capacitor 101 also in anysemiconductor device shown in FIGS. 6A, 6B, 8A, 8B, 9A, 9B, and 10A,like the semiconductor device shown in FIG. 4B. Also in that case, aneffect similar to that in the semiconductor device shown in FIG. 4B canbe attained.

Although not shown in the drawing, the transistor 115 whose firstterminal is connected to the wiring 13, second terminal is connected tothe gate of the transistor 111, and gate is connected to the wiring 12may be provided also in any semiconductor device shown in FIGS. 6A, 6B,8A, 8B, 9A, 9B, and 10A, like the semiconductor device shown in FIG. 5A.Also in that case, an effect similar to that in the semiconductor deviceshown in FIG. 5A can be attained.

Although not shown in the drawing, the wiring 14 can be omitted, and thefirst terminal of the transistor 102 may be connected to the wiring 22or the wiring 13, and the capacitor 103 one electrode of which isconnected to the wiring 12 and the other electrode of which is connectedto the other electrode of the capacitor 101 may be provided also in anysemiconductor device shown in FIGS. 6A, 6B, 8A, 8B, 9A, 9B, and 10A,like the semiconductor devices shown in FIGS. 22A and 22B. Also in thatcase, an effect similar to that in the semiconductor devices shown inFIGS. 22A and 22B can be attained.

Heretofore, the semiconductor devices whose configurations are differentfrom FIG. 6A are described.

Next, a specific example of the circuit 120 is described.

FIG. 10B is a circuit diagram of the circuit 120. The circuit 120includes a transistor 121, a transistor 122, and a transistor 123. Afirst terminal of the transistor 121 is connected to the wiring 15, asecond terminal of the transistor 121 is connected to the one electrodeof the capacitor 101, and a gate of the transistor 121 is connected tothe wiring 15. A first terminal of the transistor 122 is connected tothe wiring 13, a second terminal of the transistor 122 is connected tothe one electrode of the capacitor 101, and a gate of the transistor 122is connected to the node N1. A first terminal of the transistor 123 isconnected to the wiring 13, a second terminal of the transistor 123 isconnected to the one electrode of the capacitor 101, and a gate of thetransistor 123 is connected to the wiring 12.

The transistor 121 supplies the potential VH of the wiring 15 to the oneelectrode of the capacitor 101. The transistor 122 supplies thepotential VL1 of the wiring 13 to the one electrode of the capacitor101. The transistor 123 also supplies the potential VL1 of the wiring 13to the one electrode of the capacitor 101. The timing at which thetransistor 122 supplies the potential VL1 of the wiring 13 to the oneelectrode of the capacitor 101 is controlled by the potential of thenode N1. The timing at which the transistor 123 supplies the potentialVL1 of the wiring 13 to the one electrode of the capacitor 101 iscontrolled by the signal SE of the wiring 12.

In the period T0, since the signal SE is at the high level, thetransistor 123 is turned on. Consequently, regardless of whether thetransistor 122 is on, the potential VL1 of the wiring 13 is supplied tothe one electrode of the capacitor 101 through the transistor 123,whereby the signal IN2 becomes a low-level potential.

In the period T1, since the level of the signal SE is changed to the lowlevel, the transistor 123 is turned off. Consequently, in the case wherethe potential of the node N1 is increased and thus the transistor 122 isturned on, the potential VL1 of the wiring 13 is supplied to the oneelectrode of the capacitor 101 through the transistor 122, whereby thesignal IN2 becomes the low-level potential; in the case where thepotential of the node N1 is decreased and thus the transistor 122 isturned off, the potential VL1 of the wiring 13 is not supplied to theone electrode of the capacitor 101, whereby the signal IN2 becomes ahigh-level potential.

As shown in FIG. 10C, transistors 124, 125, and 126 may be provided inthe circuit 120 shown in FIG. 10B. A first terminal of the transistor124 is connected to the wiring 15, a second terminal of the transistor124 is connected to the one electrode of the capacitor 101, and a gateof the transistor 124 is connected to the second terminals of thetransistors 121, 122, and 123. A first terminal of the transistor 125 isconnected to the wiring 13, a second terminal of the transistor 125 isconnected to the one electrode of the capacitor 101, and a gate of thetransistor 125 is connected to the node N1. A first terminal of thetransistor 126 is connected to the wiring 13, a second terminal of thetransistor 126 is connected to the one electrode of the capacitor 101,and a gate of the transistor 126 is connected to the wiring 12. In thesemiconductor device shown in FIG. 10C, the high-level potential and thelow-level potential of the signal IN2 can be increased to VH and VL1,respectively, with a bootstrap operation.

In the circuit 120 shown in FIG. 10C, the wiring 23 may be used insteadof the wiring 15. That is, the first terminal of the transistor 121, thegate of the transistor 121, and the first terminal of the transistor 124may be connected to the wiring 23. In that case, the signal IN2 can beswitched between the high level and the low level repeatedly in theperiod Td. Accordingly, the period during which the transistors 112 and114 are on can be shortened, whereby degradation of the transistors 112and 114 can be suppressed.

Heretofore, the specific example of the circuit 120 is described.

In all or part of the period Td, the transistors 112 and 114 are turnedon when the signal IN2 is set at the high level. Consequently in thatcase, the potential of the wiring 13 is supplied to the wiring 16through the transistor 112 and to the node N1 through the transistor114. Accordingly, the potentials of the wiring 16 and the node N1 canmore surely be kept at VL1 also in the period Td.

This embodiment can be implemented in appropriate combination with anyother embodiment and the like.

Embodiment 3

In this embodiment, a shift register in which the semiconductor devicedescribed in Embodiment 2 is used as a flip-flop circuit is described.Description is made on portions different from Embodiments 1 and 2.

The shift register of this embodiment is described with reference toFIG. 11. FIG. 11 is a circuit diagram of the shift register of thisembodiment. The shift register in FIG. 11 includes N flip-flop circuits200 (N is a natural number). Among these, 1st to 3rd stage flip-flopcircuits 200 (denoted by a flip-flop circuit 200_1, a flip-flop circuit200_2, and a flip-flop circuit 200_3) are shown in FIG. 11.

In the shift register shown in FIG. 11, the semiconductor device shownin FIG. 6A is used as the flip-flop circuit 200. However, the flip-flopcircuit 200 is not limited to the semiconductor device shown in FIG. 6Aand any other semiconductor device described in Embodiment 2 can be usedas appropriate.

Connection relations in the shift register circuit shown in FIG. 11 aredescribed. The i-th stage flip-flop circuit 200 (i is any of 2 to (N−1))is connected to the i-th stage wiring 31 (denoted by a wiring 31 _(—)i), the (i−1)th stage wiring 31 (denoted by a wiring 31_(i−1)), the(i+1)th stage wiring 31 (denoted by a wiring 31_(i+1)), a wiring 32, awiring 33, a wiring 34, one of wirings 35 and 36, and a wiring 37.Specifically, in the i-the stage flip-flop circuit 200, the wiring 16 isconnected to the i-the stage wiring 31, the wiring 21 is connected tothe (i−1)th stage wiring 31, and the wiring 22 is connected to the(i+1)th stage wiring 31. Further, the wiring 15 is connected to thewiring 32, the wiring 13 is connected to the wiring 33, the wiring 14 isconnected to the wiring 34, the wiring 23 is connected to one of thewirings 35 and 36, and the wiring 12 is connected to the wiring 37. The1st stage flip-flop circuit 200 is different from the i-th stageflip-flop circuit 200 in that the wiring 21 is connected to a wiring 38.

The signal OUT is output from the wiring 31; the wiring 31 transmits thesignal OUT.

The potential VH is supplied to the wiring 32, and the wiring 32transmits the potential VH.

The potential VL1 is supplied to the wiring 33, and the wiring 33transmits the potential VL1.

The potential VL2 is supplied to the wiring 34, and the wiring 34transmits the potential VL2.

Signal CK1 is supplied to the wiring 35, and the wiring 35 transmits thesignal CK1. Signal CK2 is supplied to the wiring 36, and the wiring 36transmits the signal CK2. The signals CK1 and CK2 are similar to thesignal CK. The signals CK1 and CK2 are signals inverted from each otheror signals whose phases are different from each other.

The signal SE is input to the wiring 37, and the wiring 37 transmits thesignal SE.

Signal SP is input to the wiring 38, and the wiring 38 transmits thesignal SP. The signal SP is a start pulse of the shift register. Thesignal SP is also a digital signal whose high-level potential is VH andlow-level potential is VL1.

Next, an example of a driving method of the shift register shown in FIG.11 is described with reference to FIG. 12. FIG. 12 is an example of atiming chart for describing the driving method of the shift registershown in FIG. 11. In FIG. 12, the signal OUT of the 1st stage flip-flopcircuit 200, the signal OUT of the 2nd stage flip-flop circuit 200, andthe signal OUT of the N-th stage flip-flop circuit 200 are denoted by asignal OUT1, a signal OUT2, and a signal OUTN, respectively.

In the period T0, the signal SE is set at the high level. Consequently,each of the 1st to N-th stage flip-flop circuits 200 performs theoperation as in the period T0 described in Embodiment 2.

In the period T1, the level of the signal SE is changed to the lowlevel. Consequently, each of the 1st to N-th stage flip-flop circuits200 performs the operation as in the period T1 described in Embodiment2. Specifically, when the signal OUT of the (i−1)th stage flip-flopcircuit 200 is at the high level, the i-th stage flip-flop circuit 200performs the operation as in the period Ta described in Embodiment 2,whereby the signal OUT of the i-th stage flip-flop circuit 200 comes tobe at the low level. Then, the signals CK1 and CK2 are inverted, and thei-th stage flip-flop circuit 200 performs the operation as in the periodTb described in Embodiment 2, whereby the signal OUT of the i-th stageflip-flop circuit 200 comes to be at the high level. Then, the signalsCK1 and CK2 are inverted and the signal OUT of the (i+1)th stageflip-flop circuit 200 is changed to the high level, and the i-th stageflip-flop circuit 200 performs the operation as in the period Tcdescribed in Embodiment 2, whereby the signal OUT of the i-th stageflip-flop circuit 200 comes to be at the low level. Then, until thesignal OUT of the (i−1)th stage flip-flop circuit 200 is changed to thehigh level again, the i-th stage flip-flop circuit 200 keeps performingthe operation in the period Td described in Embodiment 2, in which thesignal OUT of the i-th stage flip-flop circuit 200 is kept at the lowlevel.

Since the semiconductor device shown in FIG. 6A is used as the flip-flopcircuit 200 in the shift register shown in FIG. 11, an effect similar tothat in the semiconductor device shown in FIG. 6A can be attained.

Heretofore, the driving method of the shift register shown in FIG. 11 isdescribed.

In the shift register shown in FIG. 11, the wiring 37 can be omitted andthe wiring 12 in each flip-flop circuit 200 may be connected to thewiring 38. In this manner, the number of wirings can be reduced. Inaddition, the offset voltage can be held periodically in the capacitor101.

In the case where the semiconductor device shown in FIG. 9A is used forthe flip-flop circuit 200, the wiring 24 is preferably connected to thewiring 36 when the wiring 23 is connected to the wiring 35. In thismanner, an increase in the number of wirings can be suppressed.

In the case where the semiconductor device shown in FIG. 9B is used forthe flip-flop circuit 200, it is preferable that the wiring 25 isconnected to the wiring 31 and the wiring 16 is connected to a load. Inthis manner, another stage flip-flop circuit 200 can be driven by thesignal OUTB of the wiring 25, which is not affected by the load, wherebythe shift register can be driven stably.

This embodiment can be implemented in appropriate combination with anyother embodiment and the like.

Embodiment 4

In this embodiment, a display device in which the shift registerdescribed in Embodiment 3 is used for a driver circuit is described.

Further, part or whole of the driver circuit can be formed over the samesubstrate as a pixel portion, whereby a system-on-panel can be obtained.

As a display element used for the display device, a liquid crystalelement (also referred to as a liquid crystal display element) or alight-emitting element (also referred to as a light-emitting displayelement) can be used. The light-emitting element includes in itscategory, an element whose luminance is controlled by a current or avoltage, and specifically includes an inorganic electroluminescent (EL)element, an organic EL element, and the like. A display medium whosecontrast is changed by an electric effect, such as electronic ink, canalso be used.

In FIG. 13A, a sealant 4005 is provided so as to surround a pixelportion 4002 provided over a first substrate 4001, and the pixel portion4002 is sealed between the first substrate 4001 and a second substrate4006. In FIG. 13A, a scan line driver circuit 4004 and a signal linedriver circuit 4003 are formed over another substrate and mounted in aregion outside a region surrounded by the sealant 4005 over the firstsubstrate 4001. Further, a variety of signals and potentials aresupplied to the signal line driver circuit 4003, the scan line drivercircuit 4004, and the pixel portion 4002 from flexible printed circuits(FPCs) 4018 a and 4018 b.

In FIGS. 13B and 13C, the sealant 4005 is provided so as to surround thepixel portion 4002 and the scan line driver circuit 4004 which areprovided over the first substrate 4001. The second substrate 4006 isprovided over the pixel portion 4002 and the scan line driver circuit4004. Thus, the pixel portion 4002 and the scan line driver circuit 4004are sealed together with the display element, by the first substrate4001, the sealing material 4005, and the second substrate 4006. In FIGS.13B and 13C, the signal line driver circuit 4003 is formed over anothersubstrate and mounted in a region outside a region surrounded by thesealant 4005 over the first substrate 4001. In FIGS. 13B and 13C, avariety of signals and potentials are supplied to the signal line drivercircuit 4003, the scan line driver circuit 4004, and the pixel portion4002 from an FPC 4018.

Although FIGS. 13B and 13C each illustrate an example in which thesignal line driver circuit 4003 is formed separately and mounted on thefirst substrate 4001, one embodiment of the present invention is notlimited to this structure. The scan line driver circuit may beseparately formed and then mounted, or only part of the signal linedriver circuit or part of the scan line driver circuit may be separatelyformed and then mounted.

A connection method of such a separately formed driver circuit is notparticularly limited; a chip on glass (COG) method, a wire bondingmethod, a tape automated bonding (TAB) method, or the like can be used.FIG. 13A illustrates an example in which the signal line driver circuit4003 and the scan line driver circuit 4004 are mounted by a COG method;FIG. 13B illustrates an example in which the signal line driver circuit4003 is mounted by a COG method; FIG. 13C illustrates an example inwhich the signal line driver circuit 4003 is mounted by a TAB method.

In addition, the display device encompasses a panel in which the displayelement is sealed, and a module in which an IC or the like including acontroller is mounted on the panel.

The display device in this specification means an image display device,a display device, or a light source (including a lighting device).Furthermore, the display device also includes the following modules inits category: a module to which a connector such as an FPC, a TAB tape,or a TCP is attached; a module having a TAB tape or a TCP at the tip ofwhich a printed wiring board is provided; and a module in which anintegrated circuit (IC) is directly mounted on a display element by aCOG method.

The pixel portion provided over the first substrate includes a pluralityof transistors.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal, aferroelectric liquid crystal, an antiferroelectric liquid crystal, orthe like is used. Such a liquid crystal material exhibits a cholestericphase, a smectic phase, a cubic phase, a chiral nematic phase, anisotropic phase, or the like, depending on conditions.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. The blue phase is one ofliquid crystal phases, which is generated just before a cholestericphase changes into an isotropic phase while the temperature of thecholesteric liquid crystal is increased. Since the blue phase appearsonly in a narrow temperature range, a liquid crystal composition inwhich 5 wt. % or more of a chiral agent is mixed is preferably used fora liquid crystal layer in order to improve the temperature range. Theliquid crystal composition which includes a liquid crystal exhibiting ablue phase and a chiral agent has a short response time of 1 msec orless, has optical isotropy, which makes the alignment process unneeded,and has a small viewing angle dependence. In addition, since thealignment film does not need to be provided, rubbing treatment is notnecessary. Consequently, electrostatic discharge caused by the rubbingtreatment can be prevented and thus defects and damage of the liquidcrystal display device in the manufacturing process can be reduced.Accordingly, productivity of the liquid crystal display device can beincreased.

The specific resistivity of the liquid crystal material is greater thanor equal to 1×10⁹ Ω·cm, preferably greater than or equal to 1×10¹¹ Ω·cm,further preferably greater than or equal to 1×10¹² Ω·cm. The specificresistivity in this specification is measured at 20° C.

The size of a storage capacitor provided in the liquid crystal displaydevice is set considering the leakage current of the transistor providedin the pixel portion or the like so that charge can be retained for apredetermined period. The size of the storage capacitor may be setconsidering the off-state current of the transistor or the like.

For the liquid crystal display device, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, anaxially symmetric aligned micro-cell (ASM) mode, an optical compensatedbirefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, anantiferroelectric liquid crystal (AFLC) mode, or the like is used.

Further, a normally black liquid crystal display device such as atransmissive liquid crystal display device utilizing a verticalalignment (VA) mode may be formed. Some examples are given as thevertical alignment mode; for example, a multi-domain vertical alignment(MVA) mode, a patterned vertical alignment (PVA) mode, an ASV mode, orthe like can be used.

This embodiment can also be applied to a VA liquid crystal displaydevice. The VA liquid crystal display device has a kind of form in whichalignment of liquid crystal molecules of a liquid crystal display panelis controlled. In the VA liquid crystal display device, liquid crystalmolecules are aligned in a vertical direction with respect to a panelsurface when no voltage is applied. Moreover, it is possible to use amethod called domain multiplication or multi-domain design, in which apixel is divided into some regions (subpixels) and molecules are alignedin different directions in their respective regions.

In the display device, a black matrix (light-blocking layer), an opticalmember (optical substrate) such as a polarizing member, a retardationmember, or an anti-reflection member, and the like are provided asappropriate. For example, circular polarization with a polarizingsubstrate and a retardation substrate may be used. In addition, abacklight, a side light, or the like may be used as a light source.

As a display method in the pixel portion, a progressive method, aninterlace method, or the like can be used. Further, color elementscontrolled in a pixel for color display are not limited to three colors:R, G, and B (R, G, and B correspond to red, green, and blue,respectively). For example, R, G, B, and W (W corresponds to white); R,G, B, and one or more of yellow, cyan, magenta, and the like; or thelike can be used. Further, the size of a display region may be differentbetween respective dots of color elements. The present invention is notlimited to the application to a display device for color display; oneembodiment of the present invention can be applied to a display devicefor monochrome display.

Alternatively, as the display element included in the display device, alight-emitting element utilizing electroluminescence can be used.Light-emitting elements utilizing electroluminescence are classifiedaccording to whether the light-emitting material is an organic compoundor an inorganic compound. In general, the former is referred to as anorganic EL element, and the latter is referred to as an inorganic ELelement.

In the organic EL element, by application of voltage to thelight-emitting element, electrons and holes are separately injected froma pair of electrodes into a layer containing a light-emitting organiccompound, and current flows. The carriers (electrons and holes) arerecombined, and thus, the light-emitting organic compound is excited.The light-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. The light-emitting element iscalled a current-excitation light-emitting element after such amechanism.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. The dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. The thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions.

Further, an electronic paper in which electronic ink is driven can beprovided as the display device. The electronic paper is also called anelectrophoretic display device (electrophoretic display) and hasadvantages in that it has the same level of readability as regularpaper, it has less power consumption than other display devices, and itcan be shaped thin and light.

Although the electrophoretic display device can have various modes, theelectrophoretic display device contains a plurality of microcapsulesdispersed in a solvent or a solute, each microcapsule containing firstparticles which are positively charged and second particles which arenegatively charged. By applying an electric field to the microcapsules,the particles in the microcapsules move in opposite directions to eachother and only the color of the particles gathering on one side isdisplayed. The first particle and the second particle each containpigment and do not move without an electric field. Further, the firstparticle and the second particle have different colors (one of them maybe colorless).

Thus, the electrophoretic display device is a display device thatutilizes a so-called dielectrophoretic effect by which a substancehaving a high dielectric constant moves to a high-electric field region.

A solution in which the above microcapsules are dispersed in a solventis referred to as electronic ink. This electronic ink can be printed ona surface of glass, plastic, cloth, paper, or the like. Furthermore, theelectronic ink also enables color display with a color filter orparticles that have a pigment.

The first particle and the second particle in the microcapsules may beformed using a single material selected from a conductive material, aninsulating material, a semiconductor material, a magnetic material, aliquid crystal material, a ferroelectric material, an electroluminescentmaterial, an electrochromic material, or a magnetophoretic material orformed using a composite material of any of these.

As the electronic paper, a display device using a twisting ball displaysystem can be used. The twisting ball display system refers to a methodin which spherical particles each colored in black and white arearranged between a first electrode layer and a second electrode layerwhich are electrode layers used for a display element, and a potentialdifference is generated between the first electrode layer and the secondelectrode layer to control orientation of the spherical particles, sothat display is performed.

The shift register described in Embodiment 3 can be applied to thedisplay device described in this embodiment, whereby a display devicewhich can operate stably even if the transistor is a depletiontransistor can be provided.

This embodiment can be implemented in appropriate combination with anyother embodiment and the like.

Embodiment 5

In this embodiment, a transistor applicable to any of the semiconductordevices described in Embodiments 1 and 2, the shift register describedin Embodiment 3, and the display device described in Embodiment 4 isdescribed.

<Oxide Semiconductor>

An oxide semiconductor is described below in detail.

An oxide semiconductor to be used preferably contains at least indium(In) or zinc (Zn). In particular, In and Zn are preferably contained. Asa stabilizer for reducing variation in electric characteristics of atransistor using the oxide semiconductor, gallium (Ga) is preferablyfurther contained. Tin (Sn) is preferably contained as a stabilizer.Hafnium (Hf) is preferably contained as a stabilizer. Aluminum (Al) ispreferably contained as a stabilizer.

As another stabilizer, one or plural kinds of lanthanoid such aslanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium(Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy),holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium(Lu) may be contained.

As the oxide semiconductor, for example, any of the following can beused: an indium oxide, a tin oxide, a zinc oxide, a two-component metaloxide such as an In—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-basedoxide, a Zn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide,or an In—Ga-based oxide, a three-component metal oxide such as anIn—Ga—Zn-based oxide (also referred to as IGZO), an In—Al—Zn-basedoxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-based oxide, anAl—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-based oxide,an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-basedoxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, anIn—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide,an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-basedoxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, or anIn—Lu—Zn-based oxide, and a four-component metal oxide such as anIn—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, anIn—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, anIn—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide.

An In—Ga—Zn-based oxide semiconductor material has sufficiently highresistance when there is no electric field and thus has a sufficientlylow off-state current. In addition, the In—Ga—Zn-based oxidesemiconductor material has a high field-effect mobility. Further, in atransistor using an In—Sn—Zn-based oxide semiconductor material, thefield-effect mobility can be three times or more as high as that of atransistor using the In—Ga—Zn-based oxide semiconductor material, andthe threshold voltage is likely to be positive. These semiconductormaterials are appropriate examples of a material that can be used for atransistor included in a semiconductor device according to oneembodiment of the present invention

For example, the “In—Ga—Zn-based oxide” means an oxide containing In,Ga, and Zn as its main components and there is no particular limitationon the ratio of In:Ga:Zn. The In—Ga—Zn-based oxide may contain a metalelement other than the In, Ga, and Zn.

Alternatively, a material represented by InMO₃(ZnO), (m>0 and m≠aninteger) may be used as the oxide semiconductor. Note that M representsone or more metal elements selected from Ga, Fe, Mn, and Co. Furtheralternatively, as the oxide semiconductor, a material represented byIn₃SnO₅(ZnO)_(n) (n>0 and n=an integer) may be used.

For example, an In—Ga—Zn-based oxide with an atomic ratio ofIn:Ga:Zn=1:1:1 (=1/3:1/3:1/3) or In:Ga:Zn=2:2:1 (=2/5:2/5:1/5), or anyof oxides whose composition is in the neighborhood of the abovecompositions can be used. Alternatively, an In—Sn—Zn-based oxide with anatomic ratio of In:Sn:Zn=1:1:1 (=1/3:1/3:1/3), In:Sn:Zn=2:1:3(=1/3:1/6:1/2), or In:Sn:Zn=2:1:5 (=1/4:1/8:5/8), or any of oxides whosecomposition is in the neighborhood of the above compositions may beused.

However, without limitation to the materials given above, a materialwith an appropriate composition may be used depending on neededsemiconductor characteristics (e.g., mobility, threshold voltage, andvariation). In order to realize the needed semiconductorcharacteristics, it is preferable that the carrier density, the impurityconcentration, the defect density, the atomic ratio between a metalelement and oxygen, the interatomic distance, the density, and the likebe set to appropriate values.

For example, a high mobility can be provided relatively easily with theIn—Sn—Zn-based oxide, whereas the mobility can be increased by reducingthe defect density in the bulk also with the In—Ga—Zn-based oxide.

For example, the case where the composition of an oxide having an atomicratio of In:Ga:Zn=a:b:c (a+b+c=1) is in the neighborhood of thecomposition of an oxide having an atomic ratio of In:Ga:Zn=A:B:C(A+B+C=1) means that a, b, and c satisfy the following relation:(a−A)²+(b−B)²+(c−C)²≦r², and r may be 0.05, for example. The same can beapplied to other oxides.

Further, it is preferable that impurities such as moisture and hydrogen,which form an electron donor (donor), be reduced, so that an oxidesemiconductor layer can be highly purified. Specifically, theconcentration of hydrogen in the highly-purified oxide semiconductorlayer that is measured by secondary ion mass spectrometry (SIMS) is5×10¹⁹/cm³ or less, preferably 5×10¹⁸/cm³ or less, further preferably5×10¹⁷/cm³ or less, still further preferably 1×10¹⁶/cm³ or less. Thecarrier density of the oxide semiconductor layer measured by Hall effectmeasurement is less than 1×10¹⁴/cm³, preferably less than 1×10¹²/cm³,further preferably less than 1×10¹¹/cm³.

Here, an analysis on the hydrogen concentration of the oxidesemiconductor layer is mentioned. The hydrogen concentration of thesemiconductor layer is measured by secondary ion mass spectrometry. Itis known that it is difficult, in principle, to obtain correct data inthe proximity of a top surface of a sample or in the proximity of aninterface between stacked layers formed of different materials by theSIMS analysis. Thus, in the case where the distribution of theconcentration of hydrogen in the layer in a thickness direction isanalyzed by SIMS, an average value is obtained in a region of the layerin which the concentration is not greatly changed and is keptsubstantially the same value, and is employed as the hydrogenconcentration. However, in the case where the thickness of the layer issmall, such a region where the concentration is kept substantially thesame value cannot be found in some cases due to the influence of theconcentration of hydrogen in an adjacent layer. In that case, themaximum value or the minimum value of the hydrogen concentration in theregion of the layer is employed as the hydrogen concentration of thelayer. Further, in the case where a mountain-shaped peak having themaximum value or a valley-shaped peak having the minimum value does notappear in the region of the layer, the value at an inflection point isemployed as the hydrogen concentration.

In the case where the oxide semiconductor layer is formed by asputtering method, it is important to reduce not only the hydrogenconcentration of a target but also water and hydrogen in a chamber, asmuch as possible. Specifically, the following are effective: inside ofthe chamber is baked before the deposition; the water and hydrogenconcentrations in a gas introduced in the chamber are reduced; andcounter flow of an exhaust system, from which a gas in the chamber isexhausted, is prevented.

The oxide semiconductor may be either single crystal ornon-single-crystal. In the latter case, the oxide semiconductor may beeither amorphous or polycrystal. Further, the oxide semiconductor mayhave either an amorphous structure including a portion havingcrystallinity or a non-amorphous structure.

In an oxide semiconductor in an amorphous state, a flat surface can beobtained with relative ease, so that when a transistor is manufacturedwith the use of the oxide semiconductor, interface scattering can bereduced, and thus relatively high mobility can be obtained with relativeease.

On the other hand, in an oxide semiconductor having crystallinity,defects in the bulk can be further reduced and when the surface flatnessis improved, mobility higher than that of the oxide semiconductor layerin an amorphous state can be obtained. To improve the surface flatness,the oxide semiconductor is preferably formed on a flat surface;specifically, the oxide semiconductor may be formed on a surface with anaverage surface roughness (Ra) of less than or equal to 1 nm, preferablyless than or equal to 0.3 nm, further preferably less than or equal to0.1 nm.

Note that R_(a) is obtained by three-dimension expansion of a centerline average roughness that is defined by JIS B 0601 so as to be appliedto a plane, and can be expressed as an “average value of the absolutevalues of deviations from a reference surface to a specified surface”and is defined by the formula below.

$\begin{matrix}{{Ra} = {\frac{1}{S_{0}}{\int_{y_{1}}^{y_{2}}{\int_{x_{1}}^{x_{2}}{{{{f\left( {x,y} \right)} - Z_{0}}}\ {x}\ {y}}}}}} & \left\lbrack {{FORMULA}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In the above formula, S₀ represents the area of a plane to be measured(a rectangular region which is defined by four points at coordinates(x₁, y₁), (x₁, y₂), (x₂, y₁), and (x₂, y₂)), and Z₀ represents theaverage height of the plane to be measured. The average surfaceroughness Ra can be measured with an atomic force microscope (AFM).

The oxide semiconductor film is in a single crystal state, apolycrystalline (also referred to as polycrystal) state, an amorphousstate, or the like.

The oxide semiconductor film is preferably a c-axis aligned crystallineoxide semiconductor (CAAC-OS) film.

The CAAC-OS film is not either complete single crystal or completeamorphous. The CAAC-OS film is an oxide semiconductor film with acrystal-amorphous mixed phase structure where crystal parts andamorphous parts are included in an amorphous phase. Note that in manycases, the crystal part has a size fits inside a cube whose side is lessthan 100 nm. From an observation image obtained with a transmissionelectron microscope (TEM), a boundary between the amorphous part and thecrystal part in the CAAC-OS film is not clear. Further, with the TEM, agrain boundary in the CAAC-OS film is not found; thus, in the CAAC-OSfilm, a decrease in electron mobility due to the grain boundary issuppressed.

In each of the crystal parts included in the CAAC-OS film, a c-axis isaligned in a direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a top surface of theCAAC-OS film, triangular or hexagonal atomic arrangement which is seenfrom the direction perpendicular to the a-b plane is formed, and metalatoms are arranged in a layered manner or metal atoms and oxygen atomsare arranged in a layered manner when seen from the directionperpendicular to the c-axis. The directions of the a-axis and the b-axisof the crystal part may be different among the crystal parts. In thisspecification, the simple expression “perpendicular” encompasses a rangefrom 85° to 95°; the simple expression “parallel” encompasses a rangefrom −5° to 5°.

In the CAAC-OS film, distribution of the crystal parts is notnecessarily uniform. For example, in the case where crystal growthproceeds from a top surface side of the oxide semiconductor film informing the CAAC-OS film, the proportion of crystal parts in thevicinity of the top surface of the oxide semiconductor film is higherthan that in the vicinity of the surface where the oxide semiconductorfilm is formed in some cases. Further, in the case where an impurity isadded to the CAAC-OS film, the crystal part in a region to which theimpurity is added becomes amorphous in some cases.

Since the c-axes of the crystal parts included in the CAAC-OS film arealigned in the direction parallel to the normal vector of the surfacewhere the CAAC-OS film is formed or the normal vector of the top surfaceof the CAAC-OS film, the directions of the c-axes may be different fromeach other depending on the shape of the CAAC-OS film (thecross-sectional shape of the surface where the CAAC-OS film is formed orthe cross-sectional shape of the top surface of the CAAC-OS film). Thedirection of the c-axis of the crystal part is the direction parallel tothe normal vector of the surface where the CAAC-OS film is formed or thenormal vector of the top surface of the CAAC-OS film as it is deposited.The crystal part is formed by deposition or by performing treatment forcrystallization such as heat treatment after deposition.

With the use of the CAAC-OS film in the transistor, change in theelectric characteristics of the transistor due to irradiation withvisible light or ultraviolet light can be decreased. Thus, thetransistor has high reliability.

Part of oxygen included in the oxide semiconductor film may besubstituted with nitrogen.

Note that the proportion of oxygen gas in an atmosphere is preferablyhigh when the CAAC-OS film is deposited by a sputtering method. Forexample, in the case of a sputtering method in a mixed gas atmosphere ofargon and oxygen, the proportion of oxygen gas is preferably 30% ormore, further preferably 40% or more. This is because oxygen is suppliedfrom the atmosphere and promotes the crystallization of the CAAC.

Further, in the case where the CAAC-OS film is deposited by a sputteringmethod, a substrate over which the CAAC-OS film is deposited ispreferably heated to 150° C. or higher, further preferably to 170° C. orhigher. This is because the higher the substrate temperature is, themore the crystallization of the CAAC is promoted.

Further, after being subjected to heat treatment in a nitrogenatmosphere or in vacuum, the CAAC-OS film is preferably subjected toheat treatment in an oxygen atmosphere or a mixed atmosphere of oxygenand another gas. This is because oxygen vacancies due to the former heattreatment can be repaired by oxygen supplied from the atmosphere in thelatter heat treatment.

Further, the film surface on which the CAAC-OS film (deposition surface)is deposited is preferably flat. This is because roughness of thedeposition surface leads to generation of grain boundaries in theCAAC-OS film because the c-axis approximately perpendicular to thedeposition surface exists in the CAAC-OS film. For this reason, thedeposition surface is preferably subjected to planarization such aschemical mechanical polishing (CMP) before the CAAC-OS film isdeposited. The average roughness of the deposition surface is preferably0.5 nm or less, further preferably 0.3 nm or less.

Next, examples of a crystal structure of the CAAC-OS film are describedin detail with reference to FIGS. 14A to 14E, FIGS. 15A to 15C, andFIGS. 16A to 16C. Unless otherwise specified, the upward directioncorresponds to the c-axis direction and a plane perpendicular to thec-axis direction corresponds to the a-b plane in FIGS. 14A to 14E, FIGS.15A to 15C, and FIGS. 16A to 16C. The simple expressions of “upper half”and “lower half” refer to an upper half above the a-b plane and a lowerhalf below the a-b plane (an upper half and a lower half with respect tothe a-b plane), respectively. Further, in FIGS. 14A to 14E, circled Osurrounded represents tetracoordinate O and double-circled O representstricoordinate O.

FIG. 14A illustrates a structure including one hexacoordinate In atomand six tetracoordinate oxygen atoms (hereinafter referred to astetracoordinate O) proximate to the In atom. Here, a structure includingone metal atom and oxygen atoms proximate thereto is referred to as asmall group. The structure in FIG. 14A is actually an octahedralstructure, but is illustrated as a planar structure for simplicity. Notethat three tetracoordinate O exist in each of an upper half and a lowerhalf in FIG. 14A. Electric charge of the small group illustrated in FIG.14A is 0.

FIG. 14B illustrates a structure including one pentacoordinate Ga, threetricoordinate oxygen atoms (hereinafter referred to as tricoordinate O)proximate to the Ga, and two tetracoordinate O proximate to the Ga. Allthe tricoordinate O exist on the a-b plane. One tetracoordinate O existsin each of an upper half and a lower half in FIG. 14B. An In atom canalso have the structure illustrated in FIG. 14B because the In can havefive ligands. Electric charge of the small group illustrated in FIG. 14Bis 0.

FIG. 14C illustrates a structure including one tetracoordinate Zn andfour tetracoordinate O proximate to the Zn. In FIG. 14C, onetetracoordinate O exists in an upper half and three tetracoordinate Oexist in a lower half; alternatively, three tetracoordinate O may existin the upper half and one tetracoordinate O may exist in the lower half.Electric charge of the small group illustrated in FIG. 14C is 0.

FIG. 14D illustrates a structure including one hexacoordinate Sn and sixtetracoordinate O proximate to the Sn atom. In FIG. 14D, threetetracoordinate O exist in each of an upper half and a lower half.Electric charge of the small group illustrated in FIG. 14D is +1.

FIG. 14E illustrates a small group including two Zn. In FIG. 14E, onetetracoordinate O exists in each of an upper half and a lower half.Electric charge of the small group illustrated in FIG. 14E is −1.

Here, a plurality of small groups is collectively referred to a mediumgroup, and a plurality of medium groups is collectively referred to as alarge group (also referred to as a unit cell).

Here, a rule of bonding between the small groups is described below. InFIG. 14A, the three O in the upper half with respect to thehexacoordinate In each have three proximate In in the downwarddirection, and the three O in the lower half each have three proximateIn in the upward direction. In FIG. 14B, the one O in the upper halfwith respect to the pentacoordinate Ga has one proximate Ga in thedownward direction, and the one O in the lower half has one proximate Gain the upward direction. In FIG. 14C, the one O in the upper half withrespect to the tetracoordinate Zn has one proximate Zn in the downwarddirection, and the three O in the lower half each have three proximateZn in the upward direction. In this manner, the number oftetracoordinate O in the upper half with respect to a metal atom isequal to the number of metal atoms proximate thereto in the downwarddirection, and the number of tetracoordinate O in the lower half withrespect to the metal atom is equal to the number of metal atomsproximate thereto in the upward direction. Since the coordination numberof the tetracoordinate O is 4, the sum of the number of the proximatemetal atoms in the downward direction and the number of the proximatemetal atoms in the upward direction is 4. Accordingly, when the sum ofthe number of tetracoordinate O in the upper half with respect to ametal atom and the number of tetracoordinate O in the lower half withrespect to another metal atom is 4, the two small groups including themetal atoms can be bonded. The reason is described below. For example,in the case where the hexacoordinate metal (In or Sn) atom is bondedthrough three tetracoordinate O in the lower half with respect to thehexacoordinate metal atom, the hexacoordinate metal atom is bonded to apentacoordinate metal (Ga or In) atom or a tetracoordinate metal (Zn)atom.

A metal atom whose coordination number is 4, 5, or 6 is bonded toanother metal atom through tetracoordinate O in the c-axis direction. Inaddition to the above, a medium group can be formed by bonding aplurality of small groups so that the total electric charge of thelayered structure is 0.

FIG. 15A illustrates a model of a medium group included in a layeredstructure of an In—Sn—Zn—O-based system. FIG. 15B illustrates a largegroup consisting of three medium groups. FIG. 15C illustrates an atomicarrangement in the layered structure in FIG. 15B when observed from thec-axis direction.

In FIG. 15A, tricoordinate O is omitted for simplicity, and with respectto tetracoordinate O, only the number thereof is illustrated; forexample, three tetracoordinate O existing in each of an upper half and alower half with respect to Sn are denoted by circled 3. Similarly, inFIG. 15A, one tetracoordinate O existing in each of an upper half and alower half with respect to In is denoted by circled 1. FIG. 15A alsoillustrates Zn proximate to one tetracoordinate O in the lower half andthree tetracoordinate O in the upper half, and Zn proximate to onetetracoordinate O atom in the upper half and three tetracoordinate O inthe lower half.

In the medium group included in the layered structure of theIn—Sn—Zn—O-based system in FIG. 15A, in the order starting from the top,Sn proximate to three tetracoordinate O in each of the upper half andthe lower half is bonded to In proximate to one tetracoordinate O ineach of the upper half and the lower half, the In is bonded to Znproximate to three tetracoordinate O in the upper half, the Zn is bondedto In proximate to three tetracoordinate O in each of the upper half andthe lower half through one tetracoordinate O in the lower half withrespect to the Zn, the In is bonded to a small group that includes twoZn atoms and is proximate to one tetracoordinate O in the upper half,and the small group is bonded to Sn proximate to three tetracoordinate Oin each of the upper half and the lower half through one tetracoordinateO in the lower half with respect to the small group. A plurality of suchmedium groups is bonded to constitute the large group.

Here, electric charge for one bond of one tricoordinate O and electriccharge for one bond of one tetracoordinate O can be assumed to be −0.667and −0.5, respectively. For example, electric charge of (hexacoordinateor pentacoordinate) In, electric charge of (tetracoordinate) Zn, andelectric charge of (pentacoordinate or hexacoordinate) Sn are +3, +2,and +4, respectively. Accordingly, electric charge in a small groupincluding Sn is +1. Therefore, electric charge of −1, which cancels +1,is needed to form a layered structure including Sn. As a structurehaving electric charge of −1, the small group including two Zn asillustrated in FIG. 14E can be given. For example, with one small groupincluding two Zn, electric charge of one small group including Sn can becancelled, so that the total electric charge of the layered structurecan become 0.

An In—Sn—Zn—O-based crystal (In₂SnZn₃O₈) can be obtained by repeatingthe large group illustrated in FIG. 15B. A layered structure of theIn—Sn—Zn—O-based crystal thus obtained can be expressed as a compositionformula, In₂SnZn₂O₇(ZnO)_(m) (m is 0 or a natural number).

The above-described rule also applies to the following oxides: afour-component metal oxide, such as an In—Sn—Ga—Zn-based oxide; athree-component metal oxide, such as an In—Ga—Zn-based oxide (alsoreferred to as IGZO), an In—Al—Zn-based oxide, a Sn—Ga—Zn-based oxide,an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-basedoxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, anIn—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide,an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-basedoxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, anIn—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide,or an In—Lu—Zn-based oxide; a two-component metal oxide, such as anIn—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-based oxide, aZn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, or anIn—Ga-based oxide; and the like.

As an example, FIG. 16A illustrates a model of a medium group includedin a layered structure of an In—Ga—Zn—O-based system.

In the medium group included in the layered structure of theIn—Ga—Zn—O-based system in FIG. 16A, in the order starting from the top,In proximate to three tetracoordinate O in each of the upper half andthe lower half is bonded to Zn proximate to one tetracoordinate O in theupper half, the Zn is bonded to Ga proximate to one tetracoordinate O ineach of the upper half and the lower half through three tetracoordinateO in the lower half with respect to the Zn, and the Ga is bonded to Inproximate to three tetracoordinate O in each of the upper half and thelower half through one tetracoordinate O in the lower half with respectto the Ga. A plurality of such medium groups is bonded to constitute.

FIG. 16B illustrates a large group consisting of three medium groups.FIG. 16C illustrates an atomic arrangement in the layered structure inFIG. 16B when observed from the c-axis direction.

Here, electric charge of (hexacoordinate or pentacoordinate) In,electric charge of (tetracoordinate) Zn, and electric charge of(pentacoordinate) Ga are +3, +2, +3, respectively, and thus electriccharge of a small group including any of In, Zn, and Ga becomes 0. As aresult, the total electric charge of a medium group consisting of acombination of such small groups is always 0.

In order to form the layered structure of the In—Ga—Zn—O-based system, alarge group may also be formed using a medium group in which thearrangement of In, Ga, and Zn is different from that in FIG. 16A.

<Transistor Whose Channel is Formed in Oxide Semiconductor Layer>

A transistor whose channel is formed in an oxide semiconductor layer isdescribed with reference to FIGS. 17A to 17D. FIGS. 17A to 17D areschematic cross-sectional views each illustrating an example of thestructure of the transistor.

The transistor illustrated in FIG. 17A includes a conductive layer601(a), an insulating layer 602(a), an oxide semiconductor layer 603(a),a conductive layer 605 a(a), a conductive layer 605 b(a), an insulatinglayer 606(a), and a conductive layer 608(a).

The conductive layer 601(a) is provided over an element formation layer600(a).

The insulating layer 602(a) is provided over the conductive layer601(a).

The oxide semiconductor layer 603(a) overlaps with the conductive layer601(a) with the insulating layer 602(a) provided therebetween.

The conductive layer 605 a(a) and the conductive layer 605 b(a) areprovided over the oxide semiconductor layer 603(a) and are electricallyconnected to the oxide semiconductor layer 603(a).

The insulating layer 606(a) is provided over the oxide semiconductorlayer 603(a), the conductive layer 605 a(a), and the conductive layer605 a(b).

The conductive layer 608(a) overlaps with the oxide semiconductor layer603(a) with the insulating layer 606(a) provided therebetween.

Both of the conductive layer 601(a) and the conductive layer 608(a) isnot necessarily provided. When the conductive layer 608(a) is notprovided, the insulating layer 606(a) is not necessarily provided.

The transistor illustrated in FIG. 17B includes a conductive layer601(b), an insulating layer 602(b), an oxide semiconductor layer 603(b),a conductive layer 605 a(b), a conductive layer 605 b(b), an insulatinglayer 606(b), and a conductive layer 608(b).

The conductive layer 601(b) is provided over an element formation layer600(b).

The insulating layer 602(b) is provided over the conductive layer601(b).

The conductive layer 605 a(b) and the conductive layer 605 b(b) are eachprovided over part of the insulating layer 602(b).

The oxide semiconductor layer 603(b) is provided over the conductivelayer 605 a(b) and the conductive layer 605 b(b), and is electricallyconnected to the conductive layer 605 a(b) and the conductive layer 605b(b). The oxide semiconductor layer 603(b) overlaps with the conductivelayer 601(b) with the insulating layer 602(b) provided therebetween.

The insulating layer 606(b) is provided over the oxide semiconductorlayer 603(b), the conductive layer 605 a(b), and the conductive layer605 b(b).

The conductive layer 608(b) overlaps with the oxide semiconductor layer603(b) with the insulating layer 606(b) provided therebetween.

Both of the conductive layer 601(b) and the conductive layer 608(b) isnot necessarily provided. When the conductive layer 608(b) is notprovided, the insulating layer 606(b) is not necessarily provided.

The transistor illustrated in FIG. 17C includes a conductive layer601(c), an insulating layer 602(c), an oxide semiconductor layer 603(c),a conductive layer 605 a(c), and a conductive layer 605 b(c).

The oxide semiconductor layer 603(c) includes a region 604 a(c) and aregion 604 b(c). The region 604 a(c) and the region 604 b(c) areprovided apart from each other and dopants are added thereto. A regionbetween the region 604 a(c) and the region 604 b(c) is a channelformation region. The oxide semiconductor layer 603(c) is provided overan element formation layer 600(c). The region 604 a(c) and the region604 b(c) are not necessarily provided.

The conductive layer 605 a(c) and the conductive layer 605 b(c) areprovided over and electrically connected to the oxide semiconductorlayer 603(c). The sides of the conductive layer 605 a(c) and theconductive layer 605 b(c) are tapered.

The conductive layer 605 a(c) overlaps with part of the region 604 a(c);however, one embodiment of the present invention is not limited thereto.Overlap between the conductive layer 605 a(c) and part of the region 604a(c) can lead to a reduction in the resistance between the conductivelayer 605 a(c) and the region 604 a(c). An entire region of the oxidesemiconductor layer 603(c) which overlaps with the conductive layer 605a(c) may form the region 604 a(c).

The conductive layer 605 b (c) overlaps with part of the region 604b(c); however, one embodiment of the present invention is not limitedthereto. Overlap between the conductive layer 605 b(c) and part of theregion 604 b(c) can lead to a reduction in the resistance between theconductive layer 605 b(c) and the region 604 b(c). An entire region ofthe oxide semiconductor layer 603(c) which overlaps with the conductivelayer 605 b(c) may form the region 604 b(c).

The insulating layer 602(c) is provided over the oxide semiconductorlayer 603(c), the conductive layer 605 a(c), and the conductive layer605 b(c).

The conductive layer 601(c) overlaps with the oxide semiconductor layer603(c) with the insulating layer 602(c) provided therebetween. A regionof the oxide semiconductor layer 603(c) which overlaps with theconductive layer 601(c) with the insulating layer 602(c) providedtherebetween is a channel formation region.

The transistor illustrated in FIG. 17D includes a conductive layer601(d), an insulating layer 602(d), an oxide semiconductor layer 603(d),a conductive layer 605 a(d), and a conductive layer 605 b(d).

The conductive layer 605 a(d) and the conductive layer 605 b(d) areprovided over an element formation layer 600(d). The sides of theconductive layer 605 a(d) and the conductive layer 605 b(d) are tapered.

The oxide semiconductor layer 603(d) includes a region 604 a(d) and aregion 604 b(d). The region 604 a(d) and the region 604 b(d) areprovided apart from each other and dopants are added thereto. A regionbetween the region 604 a(d) and the region 604 b(d) is a channelformation region. For example, the oxide semiconductor layer 603(d) isprovided over the conductive layer 605 a(d), the conductive layer 605b(d), and the element formation layer 600(d), and is electricallyconnected to the conductive layer 605 a(d) and the conductive layer 605b(d). The region 604 a(d) and the region 604 b(d) are not necessarilyprovided.

The region 604 a(d) is electrically connected to the conductive layer605 a(d).

The region 604 b(d) is electrically connected to the conductive layer605 b(d).

The insulating layer 602(d) is provided over the oxide semiconductorlayer 603(d).

The conductive layer 601(d) overlaps with the oxide semiconductor layer603(d) with the insulating layer 602(d) provided therebetween. A regionof the oxide semiconductor layer 603(d) which overlaps with theconductive layer 601(d) with the insulating layer 602(d) providedtherebetween is a channel formation region.

Next, each component illustrated in FIGS. 17A to 17D is described.

An insulating layer, a substrate having an insulating surface, or thelike can be used as the element formation layer 600(a), 600(b), 600(c),600(d). Further, a layer over which an element is formed in advance canalso be used as the element formation layer 600(a), 600(b), 600(c),600(d).

The conductive layer 601(a), 601(b), 601(c), 601(d) functions as a gateof the transistor. A layer functioning as the gate of the transistor canbe also referred to as a gate electrode or a gate wiring.

As the conductive layer 601(a), 601(b), 601(c), 601(d), it is possibleto use, for example, a layer of a metal material such as molybdenum,magnesium, titanium, chromium, tantalum, tungsten, aluminum, copper,neodymium, or scandium or an alloy material containing any of thesematerials as a main component. The conductive layer 601(a), 601(b),601(c), 601(d) can also be formed by stacking layers of materials whichcan be applied to the conductive layer 601(a), 601(b), 601(c), 601(d).

The insulating layer 602(a), 602(b), 602(c), 602(d) functions as a gateinsulating layer of the transistor.

As the insulating layer 602(a), 602(b), 602(c), 602(d), it is possibleto use, for example, a silicon oxide layer, a silicon nitride layer, asilicon oxynitride layer, a silicon nitride oxide layer, an aluminumoxide layer, an aluminum nitride layer, an aluminum oxynitride layer, analuminum nitride oxide layer, a hafnium oxide layer, or a lanthanumoxide layer. The insulating layer 602(a), 602(b), 602(c), 602(d) canalso be formed by stacking layers of materials which can be applied tothe insulating layer 602(a), 602(b), 602(c), 602(d).

Alternatively, as the insulating layer 602(a), 602(b), 602(c), 602(d),an insulating layer of a material containing an element that belongs toGroup 13 of the periodic table and oxygen can be used, for example. Forexample, in the case where the oxide semiconductor layer 603(a), 603(b),603(c), 603(d) contain an element belonging to Group 13, an insulatinglayer containing an element belonging to Group 13 can be used as aninsulating layer which is in contact with the oxide semiconductor layer603(a), 603(b), 603(c), 603(d), whereby an interface state between theinsulating layer and the oxide semiconductor layer can be improved.

Examples of the material containing an element that belongs to Group 13and oxygen include gallium oxide, aluminum oxide, aluminum galliumoxide, and gallium aluminum oxide. Aluminum gallium oxide refers to asubstance in which the amount of aluminum is larger than that of galliumin atomic percent, and gallium aluminum oxide refers to a substance inwhich the amount of gallium is larger than or equal to that of aluminumin atomic percent. For example, a material represented by Al₂O_(x)(x=3+α, where α is larger than 0 and smaller than 1), Ga₂O_(x) (x=3+α,where α is larger than 0 and smaller than 1), or Ga_(x)Al_(2-x)O_(3+α)(x is larger than 0 and smaller than 2 and α is larger than 0 andsmaller than 1) can be used.

The insulating layer 602(a), 602(b), 602(c), 602(d) can also be formedby stacking layers of materials which can be applied to the insulatinglayer 602(a), 602(b), 602(c), 602(d). For example, the insulating layer602(a), 602(b), 602(c), 602(d) can be formed of stacked layerscontaining gallium oxide represented by Ga₂O_(x). Further, theinsulating layer 602(a), 602(b), 602(c), 602(d) may be formed of a stackof an insulating layer containing gallium oxide represented by Ga₂O_(x)and an insulating layer containing aluminum oxide represented byAl₂O_(x).

Further, when the channel length of the transistor is 30 nm, thethickness of the oxide semiconductor layer 603(a), 603(b), 603(c),603(d) may be about 5 nm In that case, a short-channel effect of thetransistor can be suppressed by using a CAAC oxide semiconductor layeras the oxide semiconductor layer 603(a), 603(b), 603(c), 603(d).

Dopants imparting n-type or p-type conductivity are added to the region604 a(c), 604 b(c), 604 a(d), 604 b(d), and the region function as asource or a drain of the transistor. As the dopants, for example, one ormore of elements of Group 13 in the periodic table (e.g., boron),elements of Group 15 in the periodic table (e.g., nitrogen, phosphorus,and arsenic), and rare gas elements (e.g., helium, argon, and xenon) canbe used. A region functioning as a source of the transistor is alsoreferred to as a source region, and a region functioning as a drain ofthe transistor is also referred to as a drain region. Since addition ofthe dopants to the region 604 a(c), 6046(c), 604 a(d), 604 b(d) leads toa reduction in the contact resistance with the conductive layer, thetransistor can be downsized.

The conductive layers 605 a(a) to 605 a(d) function of a source and adrain of the transistor, and the conductive layers 605 b(a) and 605 b(d)function as a source and a drain of the transistor. A layer functioningas a source of the transistor is also referred to as a source electrodeor a source wiring, and a layer functioning as a drain of the transistoris also referred to as a drain electrode or a drain wiring.

As the conductive layer 605 a(a), 605 a(b), 605 a(c), 605 a(d), 605b(a), 605 b(b), 605 b(c), 605 b(d), for example, a layer of a metalmaterial such as aluminum, magnesium, chromium, copper, tantalum,titanium, molybdenum, or tungsten, or an alloy material which containsany of the above metal materials as a main component can be used. Forexample, the conductive layer 605 a(a), 605 a(b), 605 a(c), 605 a(d),605 b(a), 605 b(b), 605 b(c), 605 b(d) can be formed using a layer of analloy material containing copper, magnesium, and aluminum. Theconductive layer 605 a(a), 605 a(b), 605 a(c), 605 a(d), 605 b(a), 605b(b), 605 b(c), 605 b(d) can also be formed by stacking layers ofmaterials which can be applied to the conductive layer 605 a(a), 605a(b), 605 a(c), 605 a(d), 605 b(a), 605 b(b), 605 b(c), 605 b(d). Forexample, the conductive layer 605 a(a), 605 a(b), 605 a(c), 605 a(d),605 b(a), 605 b(b), 605 b(c), 605 b(d) can be formed by stacking a layerof an alloy material containing copper, magnesium, and aluminum and alayer containing copper.

As the conductive layer 605 a(a), 605 a(b), 605 a(c), 605 a(d), 605b(a), 605 b(b), 605 b(c), 605 b(d), a layer containing conductive metaloxide can also be used. Examples of the conductive metal oxide areindium oxide, tin oxide, zinc oxide, an oxide of indium and tin, and anoxide of indium and zinc. The conductive metal oxide applicable to theconductive layer 605 a(a), 605 a(b), 605 a(c), 605 a(d), 605 b(a), 605b(b), 605 b(c), 605 b(d) may contain silicon oxide.

As the insulating layer 606(a), 606(b), a layer of a material that canbe used for the insulating layer 602(a), 602(b), 602(c), 602(d) can beused. The insulating layer 606(a), 606(b) can be formed of stackedlayers of materials that can be used for the insulating layer 606(a),606(b). For example, the insulating layer 606(a), 606(b) may be formedof a silicon oxide layer, an aluminum oxide layer, or the like. Forexample, with application of an aluminum oxide layer to the insulatinglayer 606(a), 606(b), impurities (water) can be more prevented fromentering the oxide semiconductor layer 603(a), 603(b) and effectivelyprevent oxygen can be more prevented from being eliminated from theoxide semiconductor layer 603(a), 603(b).

The conductive layer 608(a), 608(b) functions as a gate of thetransistor. When the transistor includes both of the conductive layers601(a) and 608(a) or both of the conductive layers 601(b) and 608(b),one of the conductive layers 601(a) and 608(a) or one of the conductivelayers 601(b) and 608(b) is also referred to as a back gate, a back gateelectrode, or a back gate wiring. In this manner, a plurality ofconductive layers each functioning as a gate may be provided with thechannel formation layer provided therebetween, whereby the thresholdvoltage of the transistor can be more easily controlled.

As the conductive layer 608(a), 608(b), a layer of a material that canbe used for the conductive layer 601(a), 601(b), 601(c), 601(d) can beused, for example. The conductive layer 608(a), 608(b) may also beformed of stacked layers of materials that can be used for theconductive layer 608(a), 608(b).

Further, an insulating layer functioning as a channel protective layermay be formed of stacked layers of materials that can be used for theinsulating layer 602(a), 602(b), 602(c), 602(d).

Further, a base layer may be formed over the element formation layer600(a), 600(b), 600(c), 600(d), and the transistor may be formed overthe base layer. In that case, a layer of a material that can be used forthe insulating layer 602(a), 602(b), 602(c), 602(d) can be used as thebase layer, for example. The base layer may also be formed of stackedlayers of materials that can be used for the insulating layer 602(a),602(b), 602(c), 602(d). For example, a base layer may be formed of astack of an aluminum oxide layer and a silicon oxide layer, and therebyelimination of oxygen in the base layer through the oxide semiconductorlayer 603(a), 603(b), 603(c), 603(d) can be suppressed.

Further, the insulating layer in contact with the oxide semiconductorlayer 603(a), 603(b), 603(c), 603(d) may be formed to contain excessoxygen, whereby oxygen can be more easily supplied to the oxidesemiconductor layer 603(a), 603(b), 603(c), 603(d). Accordingly, oxygendefects in the oxide semiconductor layer 603(a), 603(b), 603(c), 603(d)and in an interface between the insulating layer and the oxidesemiconductor layer 603(a), 603(b), 603(c), 603(d) can be reduced; thus,the carrier density of the oxide semiconductor layer 603(a), 603(b),603(c), 603(d) can be more reduced. One embodiment of the presentinvention is not limited thereto. The oxide semiconductor layer 603(a),603(b), 603(c), 603(d) may be formed to contain excess oxygen in themanufacturing process, also in which case elimination of oxygen from theoxide semiconductor layer 603(a), 603(b), 603(c), 603(d) can besuppressed by the above-described insulating layer in contact with theoxide semiconductor layer 603(a), 603(b), 603(c), 603(d).

<Characteristics of Transistor Whose Channel is Formed in OxideSemiconductor Layer>

In a transistor in which an oxide semiconductor containing In, Sn, andZn as its main components is used for a channel formation region,favorable characteristics can be provided by depositing the oxidesemiconductor while heating a substrate or by performing heat treatmentafter an oxide semiconductor layer is formed. The “main component” meansthat the element is contained in composition at 5 atomic % or more.

By intentionally heating the substrate after formation of the oxidesemiconductor layer containing In, Sn, and Zn as its main components,the field-effect mobility of the transistor can be improved. Inaddition, the threshold voltage of the transistor can be shifted in thepositive direction to make the transistor normally off.

For example, FIGS. 18A to 18C each show characteristics of a transistorthat includes an oxide semiconductor layer containing In, Sn, and Zn asits main components and having a channel length L of 3 μm and a channelwidth W of 10 μm, and a gate insulating layer with a thickness of 100nm. Here, V_(d) was set at 10 V.

FIG. 18A shows characteristics of a transistor whose oxide semiconductorlayer containing In, Sn, and Zn as its main components was formed by asputtering method without heating a substrate intentionally. Thefield-effect mobility of the transistor was 18.8 cm²/Vsec. On the otherhand, when the oxide semiconductor layer containing In, Sn, and Zn asits main components is formed while heating the substrate intentionally,the field-effect mobility can be improved. FIG. 18B showscharacteristics of a transistor whose oxide semiconductor layercontaining In, Sn, and Zn as its main components was formed whileheating a substrate at 200° C. The field-effect mobility of thetransistor was 32.2 cm²/Vsec.

The field-effect mobility can be further enhanced by performing heattreatment after formation of the oxide semiconductor layer containingIn, Sn, and Zn as its main components. FIG. 18C shows characteristics ofa transistor whose oxide semiconductor layer containing In, Sn, and Znas its main components was formed by sputtering at 200° C. and thensubjected to heat treatment at 650° C. The field-effect mobility of thetransistor was 34.5 cm²/Vsec.

Such substrate heating or heat treatment acts such that hydrogen and ahydroxyl group, which are adverse impurities for an oxide semiconductor,are not included in the film or are removed from the film. That is, anoxide semiconductor can be highly purified by removing hydrogen servingas a donor impurity from the oxide semiconductor, which enables atransistor to be a normally-off transistor and enables the off-statecurrent of the transistor to be reduced to 1 aA/μm or lower. Here, theoff-state current is described per micrometer of channel width.

FIG. 19 shows a relation between the off-state current of a transistorand the inverse of substrate temperature (absolute temperature) atmeasurement. Here, for simplicity, a value (1000/T) obtained bymultiplying the inverse of substrate temperature at measurement by 1000is indicated in the horizontal axis.

As shown in FIG. 19, the off-state current was 0.1 aA/μm (1×10⁻¹⁹ A/μm)or smaller and 10 zA/μm (1×10⁻²⁰ A/μm) or smaller when the substratetemperature was 125° C. and 85° C., respectively. The proportionalrelation between the logarithm of the off-state current and the inverseof the temperature suggests that the off-state current at roomtemperature (27° C.) is 0.1 zA/mm (1×10⁻²² A/mm) or smaller. As isapparent from the above, the off-state current can be 1 aA/μm (1×10⁻¹⁸A/mm) or smaller, 100 zA/μm (1×10⁻¹⁹ A/μm) or smaller, and 1 zA/μm(1×10⁻²¹ A/μm) or smaller at 125° C., 85° C., and room temperature,respectively.

The transistor described in this embodiment can be used for thesemiconductor device described in Embodiment 1 or 2, whereby thesemiconductor device can operate stably. In particular, by using thetransistor described in this embodiment as the transistor 102, theoff-state current of the transistor 102 can be reduced; accordingly, theamount of charge leaks from the capacitor 101 can be reduced, and thusthe frequency of times of holding the offset voltage in the capacitor101 can be reduced.

This embodiment can be implemented in appropriate combination with anyother embodiment and the like.

Embodiment 6

In this embodiment, examples of an electronic device equipped with thesemiconductor device, the shift register, the display device, or thelike described in any of the above embodiments are described.

FIG. 20A shows a portable game console that includes a housing 9630, adisplay portion 9631, a speaker 9633, operation keys 9635, a connectionterminal 9636, a recording medium reading portion 9672, and the like.The portable game console illustrated in FIG. 20A can have a function ofreading a program or data stored in a recording medium to display on thedisplay portion; a function of sharing data by wireless communicationwith another portable game console; or the like. The function of theportable game console illustrated in FIG. 20A is not limited thereto,and various functions can be provided.

FIG. 20B illustrates a digital camera which includes a housing 9630, adisplay portion 9631, speakers 9633, operation keys 9635, a connectionterminal 9636, a shutter button 9676, an image receiving portion 9677,and the like. The digital camera in FIG. 20B can have a function oftaking a still image and/or a moving image, a function of automaticallyor manually correcting the taken image, a function of detecting variouskinds of data from an antenna, a function of holding the taken image orthe data detected from the antenna, a function of displaying the takenimage or the data detected from the antenna on the display portion, andthe like. The function of the digital camera illustrated in FIG. 20B isnot limited thereto, and various functions can be provided.

FIG. 20C illustrates a television set which includes a housing 9630, adisplay portion 9631, speakers 9633, operation keys 9635, a connectionterminal 9636, and the like. The television set in FIG. 20C has afunction of converting an electric wave for television into an imagesignal, a function of converting an image signal into a signal suitablefor display, a function of converting the frame frequency of an imagesignal, and the like. The function of the television set illustrated inFIG. 20C is not limited thereto, and various functions can be provided.

FIG. 20D illustrates a monitor for electronic computers (personalcomputer) (the monitor is also referred to as a PC monitor) thatincludes a housing 9630, a display portion 9631, and the like. As anexample, in the monitor in FIG. 20D, a window-type display portion 9653is provided for the display portion 9631. Note that FIG. 20D illustratesthe window-type display portion 9653 in the display portion 9631 forexplanation; another symbol such as an icon or an image may bedisplayed. In the monitor for a personal computer, an image signal isrewritten only at the time of data inputting in many cases, which ispreferable to apply the method for driving a display device in theabove-described embodiment. The function of the monitor illustrated inFIG. 20D is not limited thereto, and various functions can be provided.

FIG. 21A illustrates a computer that includes a housing 9630, a displayportion 9631, a speaker 9633, operation keys 9635, a connection terminal9636, a pointing device 9681, an external connection port 9680, and thelike. The computer illustrated in FIG. 21A can have a function ofdisplaying various kinds of data (e.g., a still image, a moving image,and a text image) on the display portion; a function of controllingprocessing by various kinds of software (programs); a communicationfunction such as wireless communication or wire communication; afunction of connecting with various computer networks by usingcommunication function; a function of transmitting or receiving variouskinds of data by using communication function; or the like. The functionof the computer illustrated in FIG. 21A is not limited thereto, andvarious functions can be provided.

FIG. 21B illustrates a mobile phone that includes a housing 9630, adisplay portion 9631, a speaker 9633, operation keys 9635, a microphone9638, and the like. The mobile phone illustrated in FIG. 21B can have afunction of displaying a variety of data (e.g., a still image, a movingimage, and a text image) on the display portion; a function ofdisplaying a calendar, date, the time, and the like on the displayportion; a function of operating or editing data displayed on thedisplay portion; a function of controlling processing by various kindsof software (programs); or the like. The function of the mobile phoneillustrated in FIG. 21B is not limited thereto, and various functionscan be provided.

FIG. 21C illustrates electronic paper (also referred to as an e-book oran e-book reader) that includes a housing 9630, a display portion 9631,operation keys 9632, and the like. The electronic paper illustrated inFIG. 21C has a function of displaying various kinds of data (e.g., astill image, a moving image, and a text image) on the display portion, afunction of displaying a calendar, a date, the time, or the like on thedisplay portion, a function of operating or editing data displayed onthe display portion, a function of controlling processing by variouskinds of software (programs), or the like. The function of theelectronic paper illustrated in FIG. 21C is not limited thereto, andvarious functions can be provided. FIG. 21D illustrates anotherelectronic paper. The electronic paper in FIG. 21D has a structure inwhich a solar battery 9651 and a battery 9652 are added to theelectronic paper in FIG. 21C. When a reflective display device is usedas the display portion 9631, the electronic paper is expected to be usedin a comparatively bright environment, in which case the structure inFIG. 21D is preferable because the solar battery 9651 can efficientlygenerate power and the battery 9652 can efficiently charge power. Notethat it is advantageous to use a lithium ion battery as the battery 9652in a reduction in size or the like.

The semiconductor device described in Embodiment 1, the semiconductordevice described in Embodiment 2, the shift register described inEmbodiment 3, or the display device described in Embodiment 4 can beapplied to any electronic device described in this embodiment, wherebythe electronic device can operate even when a transistor thereof is adepletion transistor.

This embodiment can be implemented in appropriate combination with anystructure described in any other embodiment.

This application is based on Japanese Patent Application serial no.2011-108133 filed with Japan Patent Office on May 13, 2011, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A driving method of a semiconductor device whichcomprises: a first circuit comprising a capacitor and a firsttransistor; and a second circuit comprising a second transistor, whereina gate of the second transistor is connected to the first circuit,wherein the first transistor has a same conductivity type as the secondtransistor, the driving method comprising the steps of: supplying afirst potential to a first electrode of the capacitor; supplying asecond potential to a second electrode of the capacitor through thefirst transistor, wherein the second potential is lower than the firstpotential; and making the second electrode of the capacitor into afloating state.
 3. The driving method according to claim 2, wherein anoffset voltage is held in the capacitor, and wherein a signal inaccordance with the offset voltage is input to the gate of the secondtransistor.
 4. The driving method according to claim 2, wherein at leastone of the first transistor and the second transistor comprises an oxidesemiconductor.
 5. The driving method according to claim 2, wherein thesecond circuit further comprises a third transistor, a fourthtransistor, and a fifth transistor each having the same conductivitytype as the second transistor.
 6. The driving method according to claim2, wherein the second potential is lower than a potential supplied to afirst terminal of the second transistor.
 7. A driving method of asemiconductor device which comprises: a capacitor; a first transistor;and a second transistor, wherein a first electrode of the capacitor isconnected to a first wiring, wherein a first terminal of the firsttransistor is connected to a second wiring, wherein a second electrodeof the capacitor, a second terminal of the first transistor, and thegate of the second transistor are connected to each other, wherein thefirst transistor has a same conductivity type as the second transistor,the driving method comprising the steps of: supplying a first potentialto the first electrode of the capacitor; turning on the first transistorso that a second potential is supplied to the second electrode of thecapacitor through the first transistor; and turning off the firsttransistor so that the second electrode of the capacitor is made into afloating state.
 8. The driving method according to claim 7, wherein anoffset voltage is held in the capacitor, and wherein a signal inaccordance with the offset voltage is input to the gate of the secondtransistor.
 9. The driving method according to claim 7, wherein at leastone of the first transistor and the second transistor comprises an oxidesemiconductor.
 10. The driving method according to claim 7, wherein thesemiconductor device further comprises a third transistor, a fourthtransistor, and a fifth transistor each having the same conductivitytype as the second transistor.
 11. The driving method according to claim7, wherein the second potential is lower than a potential supplied to afirst terminal of the second transistor.
 12. A driving method of asemiconductor device which comprises: capacitor; a first transistor; anda second transistor, wherein a first electrode of the capacitor isconnected to a first wiring, wherein a first terminal of the firsttransistor is connected to a second wiring, wherein a second electrodeof the capacitor, a second terminal of the first transistor, and thegate of the second transistor are connected to each other, wherein thefirst transistor has a same conductivity type as the second transistor,the driving method comprising the steps of: in a period during which anoffset voltage is held in the capacitor, supplying a first potential tothe first electrode of the capacitor; and turning on the firsttransistor so that a second potential is supplied to the secondelectrode of the capacitor through the first transistor, in a periodduring which a signal in accordance with the offset voltage is outputfrom the capacitor, turning off the first transistor so that the secondelectrode of the capacitor is made into a floating state; and outputtingthe signal in accordance with the offset voltage from the capacitor tothe gate of the second transistor.
 13. The driving method according toclaim 12, wherein at least one of the first transistor and the secondtransistor comprises an oxide semiconductor.
 14. The driving methodaccording to claim 12, wherein the semiconductor device furthercomprises a third transistor, a fourth transistor, and a fifthtransistor each having the same conductivity type as the secondtransistor.
 15. The driving method according to claim 12, wherein thesecond potential is lower than a potential supplied to a first terminalof the second transistor.